As a portrait artist with a passion for fashion and an admiration for plus-size beauty, I hope my tips below will be of interest and use. I tend to be a free-thinker and disagree with many so-called experts, so what follows is not a copy of anyone else's list of tips.
Hard and Fast Rules
There are very few hard and fast rules. Fashion 'experts' like to lay down strict rules to show off, or justify their existence. Examples of common "rules" that this author disagrees with are: "Don't wear red if you have red hair". Wrong! Even bright red clothes may work depending on the type of red hair you have. "A round face needs square glasses". Wrong! Try both and see what works for you. "Large ladies should cut their hair short". Wrong again! That is a sweeping generalisation. "Large ladies should wear dark clothes". Very wrong! Dark clothing hides shape, which is a pity if you have feminine curves, unless the dark material has a shine to it (velvet or satin). "Horizontal stripes make you look wider and vertical stripes make you look thinner". Simplistic, and largely wrong! Thin stripes make no difference. Much broader bands may have some effect, while panelled effects can make a difference if done right. As always, it depends on the shape of the lady, and horizontal breaks in colour can sometimes make the lady look slimmer or a different shape.
Every Seam On The Beam
Clothing that fits well all over, will make the outfit look good even if it is just a casual t-shirt and jeans look. Finding things that fit well, may be a problem the larger you are, and also because of the variety of proportions of larger ladies. Shoulders are an area to pay careful attention to since a lady's shoulders may be narrow relative to their torso, or else may be broad. If the seams for the shoulders are down at your elbows, or only half way across your shoulders, then it ruins the look of a nice top or dress. If you have trouble finding tops and dresses that will fit both torso and shoulders well, then try something that does not have seams over the shoulders, such as raglan or batwing sleeves.
Close Fitting, Not Tight
To paraphrase a Mae West quote, clothing should be close fitting enough to show that the wearer is a woman, but loose enough to show that she is a lady. Tight, clinging clothes do not usually look good, but the alternative should not be to go to the opposite extreme of bagginess. If you choose materials that flow gently around your curves, and have a bit of stretch to avoid any tight spot as you move, or sit down, then you can look feminine and smart. Looser flowing clothes can work well too, just as long as they are not hiding your entire shape. Popular attempts to hide a large bust under baggy blouses and shirts, only succeed in making you look larger all over and shapeless too. As dark colours tend to hide shape, a top-heavy lady may benefit from wearing a dark top that is reasonably form fitting, and a light coloured skirt or pair of pants? That would show off her curves in a subtle, fairly conservative way, rather than trying to hide too much.
Colours & Patterns
There used to be a tendency to produce very bold patterns with huge flowers or other printed designs for plus size clothing, usually on something resembling a tent! In fact, much finer printed patterns work excellently. This can work very well on fairly form fitting clothes, as the print (maybe a simple small polka-dot design?) helps to show the contours of one's curves, in a subtle way. They can also look equally pretty on looser clothes, such a blouson top or a drop-waist dress, or maybe a tunic.
Horizontal stripes (contrary to 'expert' advice) can look superb on full-figured ladies. Preferably thin stripes, or variable thickness in the more intricate designs of some striped tops. If you are keen on something that will have a slimming effect, then panelled designs might help. For example, a dark colour for each side of a top, and a pale colour for the front and back. The join between the colours needs to be well towards the sides (i.e a fairly wide panel at the front and back) for the effect to work. Strictly speaking, the effect only works when you are viewed from the front, rear or in profile, but not in three-quarter view, because in the latter view, the join between colours appears in the centre.
If your hips are large in relation to the rest of your figure, then please think of this as a feminine attribute, and take pride in your curves. A popular (and very good) approach to the larger hips, is a long (or longish) skirt that flows and swirls. These skirts are always feminine in appearance, as they swirl when you walk, but are especially good with larger hips and longer legs. For longer legs it might only need to be just below the knee, in length. For shorter legs (short relative to torso length), a lower hemline is often better, but this depends upon the whole outfit.
Pants can also look great with larger hips, but will need to be chosen carefully. Pants that taper down to the ankles are usually best, although flared ones can work if they fit well, with some tapering above the flaring. Black pants with a light or bright top, can be a good combination, depending on your preferences.
For hips that are not so large (relative to one's overall figure), then straight, pencil skirts or a variety of pants often work best. But it also depends on how small or large one's waistline is too, since with a small waist, the hips may still be very curvy. With curvy hips, swirling skirts will work well too, but shorter lengths (as well as long ones) may be viable.
If you have a well defined waist, then there are many ways to make the most of this quality. Slinky materials that flow gently around your curves. Or detailing such as a belt to draw attention to your waist. Wider belts are only recommended for the slimmer end of the plus size scale, or for those who have a long torso between bust and hips. Thinner belts worn loosely can be an excellent accessory.
If you are larger around the waistline, then your approach will depend on how high up your bustline is. An empire waist dress or tunic top may look good if your bustline is not particularly high. But can be less flattering for a higher bustline, because the fake waistline then becomes too high up to be convincing and may end up emphasising the tummy below. Experiment with different lengths for tops, as the right length for your particular shape can help a great deal.
For those ladies who are larger around the hips, long and swirling skirts tend to be the norm. But you don't have to follow established practice. I would recommend considering the hemline carefully. The hemline breaks up your height, and also the length of your legs. A very short dress breaks up your height close to half way up, and thus might make you look shorter, unless your legs are very long. A hemline at knee level, breaks up your leg length half way, and might make them look shorter. The shape of your legs influences the hemline decision, but oddly enough, this is less so with larger ladies than with slim women. That is because on a slim woman, the skirt may almost touch the knees, but a much larger lady has knees recessed into the shadows of the skirt. And because people see your legs from above (unless they are lying down!), a higher hemline will still hide more leg. The decision comes down more to the silhouette the skirt/dress forms around your hips and down to the hem.
Darts at the top of the skirt, intended to curve it into your waist, help to break up the width. Ditto for twin seams on the front and back of the skirt, which is more subtle than different colored panels. This breaking up of the width then allows a higher hemline if you wish.
For ladies who are well endowed, there are a number of possible approaches. One approach to avoid is wearing anything tent-like to try and hide one's bust. Material with a little 'give' or stretch will help, by flowing gently around those curves without showing every detail. One suggestion (already touched upon above) is a fairly close fitting stretchy top but in a dark colour. The darkness tends to hide shape, so that people's attention won't be drawn by the bust, yet the clothing won't hide one's entire figure. That is a good approach if you are fairly conservative in fashions, or don't have lots of confidence in your appearance. With more confidence, a lighter colour top, but with a dark cardigan or jacket, also works very well for a top-heavy figure. If your tummy/waistline are much smaller than your bust, then it can be good to emphasise this, provided that your bustline is reasonably high. Such as a top tucked into pants/skirt? Or a thin belt? If the bustline is lower then a longer top helps and its hem should be lower at the front - if necessary by raising the hem at the back so that the rear of the waist and curve of the bottom is not lost under loose clothing.
Another approach to 'hiding' one's bust, is to draw attention away. For example, pretty sleeves in a contrasting material or colour/print. Or if you have an hour-glass figure, an attractive belt. The belt's style can be quite subtle, but in a contrasting colour to your top.
For a bust that is small relative to one's overall figure, one can go for a hint of cleavage, if appropriate for the outfit. Or else emphasise one's other curves, and maybe experiment with raising one's apparent waistline.
Necklines & Shoulderlines
Larger ladies generally have the most beautiful shoulders, of soft gentle feminine curves! If your shoulders are very broad compared to your torso, then broader necklines often look best, such as boatneck designs. Shoulders that are fairly narrow compared to one's bust are ideal for a halter neck, but as mentioned above, one can't make hard and fast rules when so many different shapes and sizes of figure are evident. Scalloped necklines may also suit either narrower shoulders, or an outfit with slightly puffed sleeves.
A narrow neckline, such as a narrow plunging V-neck, can also work for a lady with broader shoulders if the seams for the sleeves are set widely apart. Alternatively, a simple bandeau strappy top/dress combined with a shrug or cardigan, can produce an interesting square-neckline effect, possibly with different colours for the two garments?
If you do have the confidence to show a bit of shoulder, then an off-the-shoulder top, worn on top of a strappy vest, with the straps evident, is another popular approach. A really pretty variation of this is a lace see-through top, worn over a stappy vest of the same colour.
Often dismissed as evil, painful garments (especially by well endowed full-figured ladies), they only give problems when they do not fit properly. Take my word for it that some very top heavy plus sized ladies manage to have a pain-free life as far as bras are concerned. The shoulder straps are not there to support weight and should not dig in deeply or cause pain, and there should be no back pains related to one's bust if wearing the correct bras.
A great many larger busted ladies tend to wear band sizes too large combined with cup sizes too small. Until recently they had little choice, but bra manufacturers have finally come to visit planet earth and produce most of the sizes needed. Another problem affects ladies of a very large build, especially if the tummy is large, as this can make underwired bras unsuitable. Underwired bras cannot adapt to all the shapes of the larger ladies. Traditionally, non-wired bras have tended to be unattractive, and only offering full coverage (no good for lower necklines, V-neck, etc.). Thankfully, this is another situation that is gradually improving, and some of the larger, non-wired bras are quite pretty and lower cut for greater versatility.
The moral of the story is to measure yourself accurately (or get professionally measured), and then keep in mind that different bra manufacturers have different cup sizing. For example, you might be a DDD, E, or an F cup depending on which make of bra you purchase. You can see equivalent sizing on this page: Bra cup size chart. Extra large ladies may find that following the recommended measuring systems, even if done professionally, still does not provide a good fit. This might be due to the softness of your torso, and you might find that going 1 or 2 sizes smaller (e.g. 46 instead of 48 inch band) works.
Wearing good quality shape-wear can tranform how you look in any form-fitting tops and dresses. Modern developments in manufacturing mean you can achieve that feminine hourglass figure, but remain comfortable. Larger ladies may see their extra padding as a problem, but their softness also makes them easier to mould for a very feminine and beautiful shape.
Longline tops are often recommended and are popular with curvy ladies. They usually work very well for a flattering look, but there is a potential problem to avoid. If a top is particularly long, and also loose without shape, it can give the illusion that your legs are very short. That is because it is obscuring where your waist and hips are, as well as the legs themselves. A quick fix is to add a thin belt (maybe even just a loose one), which immediately shows where the waist is, and thus gives a nod to the length of your legs. It makes all the difference!
Confidence, or lack of it, underlies one's attitude to one's own fashion. This is doubly hard to achieve for larger ladies in a world obsessed with thinness. Finding fashions that fit and suit your taste, can be a first step in building up confidence and then perhaps trying a greater variety of fashions. Also keep in mind that your reaction to people looking at you can have more to do with your self-confidence than reality. If people look, they may be admiring. - especially the men! If your bottom looks big, then many men will like that - it is a feminine attribute after all! If your upper arms are large and soft, then that too is a feminine characteristic, just as 'padding' builds up on ladies' thighs too. Legs and arms tend to taper gracefully down to the ankles/wrists. People looking at you may also be admiring your clothing and good fashion sense, which is a compliment to some of your inner qualities.
A most important aspect of style is your deportment and poise. The way you walk, stand, sit and move. Hold your head up, look cheerful, avoid slouching, and give an air of confidence even if it is only a put-on. The positive reaction from other people may then start to give you a genuine confidence. Others may treat you better, come up and chat and smile more. If anyone stares, then consider that they are probably admiring, and if they are not, then who cares? Anyone who actually says anything negative is an idiot for saying so, and their view carries no importance - especially when there may be so many out there who know that you are attractive. Keep smiling :)