The most difficult thing about dressing up is choosing clothes that'll look good on you. Even if you genuinely do have the body of a supermodel, there are clothes that will simply look bad. A good start is to work out what size you are. Catalogues have size charts that you can use to work out what size you are, and some web sites have these charts reproduced. The typical transvestite must also get a size larger top than bottom to compensate for that masculine 'V' shape.
Remember however that this isn't the end of the story - not all clothes that claim to be a size 12 are the same size. Stores cut their clothes smaller or larger depending on the market that they're selling to, so you can grow from being a teenager to middle age and apparently stay the same size. Similarly some stores that aim for the 'fuller' figure are more generous in their cut. Women can find that certain stores, brands or labels are better cut for them, and trial and error plays a big part in finding your best source for clothes. The simple rule is to keep the reciept and bag until you're sure the clothes fit.
The next trick is to choose clothes that flatter. Unless your body is like Cindy Crawford's (in which case, why are you reading this, Cindy?) you need to give the impression of a better figure than you have. To do this, you need to hide the nasty bits and provide visual clues to the figure you're aiming at. Hiding nasty bits involves loose fitting clothes; as an example, a nice baggy jumper can hide a waistline that's far bigger than you'd like and still look sexy. A jacket can give the same effect, so long as it's not tight around your middle.
Providing visual clues involves taking attention away from dimensions that don't look good, and drawing the eye to those that do. Think of those optical illusions that make straight lines look curved and long lines look short - that's what you want to do with your body. For a start, few tall women want to draw attention to their height, so vertical stripes are out, as are horizontal stripes if you are anything more than super-slim. Break up a tall body with seperates, and try to raise your waistline so that your legs do look like they go up to your armpits. Some tops have lines that follow the hourglass shape (like the boning on corsets) and these help to convince the brain that curves really are there.
You can also get curves from flared skirts and fitted dresses that'll probably look curvey even when they're hanging up. On the whole, the better quality the clothing is, the more likely it is to make you look good, so long as you're not trying to look sexy in a two inch wide strip of lycra. Cheap stores and catalogues may sell items that look good in the pictures and posters, but those images are often achieved with rolls of duct-tape and safety pins used to hold the clothes in an ideal line on a woman who's also a professional model. At the other end of the scale, professional dressmakers can make you something that obeys all the rules and is very flattering - if you don't mind the added expense. A good dressmaker will ask you back for at least one fitting before they finish the garment, and very fitted items and corsets may require a test fitting and two later fittings to get just right.
Colour also plays an important part. Black is undoubtedly the most flattering colour, as it tends to make your body look slimmer. However, colour can also draw the eye from a difficult area or outline a better shape. If you dress in a single colour, it'll tend to flatten you out - whereas contrasts will make it harder to get a straight view of your outline (and so hide a few imperfections).
Trannies also have to remember that there are bits that just aren't feminine. Muscle-bound arms are best hidden with sleeves or opera gloves if you have nice shoulders. Hide a male adams apple and colar bones with high necked tops or chokers. Ankle boots hide boney ankles, and thigh length boots hide boney knees as well (but aren't too subtle for daytime wear).
Finally, if you want to get away with it, remember the bloke who was only noticed by a police woman (and then arrested) because the officer thought it rather unusual to see a nun wearing stillettoes. The lesson to learn from this is that the best disguise is blending in - and therefore dressing appropriately. Very few women wear particulary short skirts, and stockings are not all that common. Women dress more simply during the day and wear much less noticable makeup. Even in the evening, looking like a tart will probably get you the kind of attention that leads to deep trouble - so think hard about how many women you've seen lately dressed the way you are before you venture out.
Of course, if you're staying in, all you've got to worry about is scaring your partner and house-mates. :-)