Steps

1. Stand up straight. If you improve your posture, not only do you decrease visibility of the rolls in your stomach, but you also gain confidence!! Pair that with a winning smile and you're good to go.
2. Wear proper fitting underwear. The look you're going for is seamless - no ill-fitting bras that leave back fat spilling over in bulging rolls, and try on a slip.
3. Wear long, fitting shirts. These are flattering to everybody since they seem to stretch out your torso. They should fit to flatter your body type, but please, don't immediately dash to the nearest Abercrombie and Fitch. Their shirts are too snug if you're worried about looking thinner.
4. Choose the colors carefully. Avoid bright colors and patterns; they instantly make you look bigger than you are! Dark colors make your heavy sections look small. Are you top heavy? That means switching to dark colored tops. Also, avoid horizontal stripes; these accentuate your girth! If you are bottom heavy, go for darker pants, skirts, or jeans to slim down your bottoms and legs. Finally, avoid "baggy" clothing, and wear stuff that fits you closely.
5. Match your shoes to your build. If you are short and heavy, a pair of understated platform clogs or boots can give you height, and also counterbalance your body size. If you are tall and heavy, a pair of dark colored flats or low heel pumps will make your feet look smaller without making you even taller. Avoid slender heels and sharp pointy toes; they will look small on you, and accentuate your size.
6. Wear your hair and makeup in a flattering way. People will be too distracted by your beautiful face to care about everything else.
7. Avoid the "I'm so fat" conversations. Most everybody has been involved in one of these and they only bring you down. You are most likely NOT fat. Most people don't care as much as you think they do because they are caught up in themselves. Flaunt your body the way it is. If it helps, make a list of all the people who love you and all the things you love about yourself. You first must love yourself before you love others. Confidence is key.
8. Wear clothes that are the right size. If you're a size 20, don't try and squeeze into a size 16. It will make you look bigger if everything bulges out. But at the same time, don't wear clothes that swamp you - really look at your body and wear clothes that fit properly.
9. Don't wear striped shirts, they make you appear wider. Unless they are up and down (pin stripes) which makes you appear longer and leaner
10. Emphasize the smaller part of you and camouflage your problem areas. If your waist is relatively thinner than your hips, wear form-fitting shirts and skirts that flare out. If your middle is thick and your legs are normal sized, wear looser, tunic blouses with thin, straight-legged pants. DON'T wear loose tops with loose bottoms.
11. Match your jewelry to your size. If you're a big, tall person, wear big pieces of jewelry. Think a (one!)nice chunky bracelet or a pair of chandelier earrings. A big necklace will look great too -but avoid chokers.