Mimic your body shape.
Follow what your body does naturally. For example, if you have an hourglass shape, find pieces of the same shape, and accent them with a belt.
Play up your Curves & angles.
If you have a really curvy body, flatter your shape with draping fabrics. It will fall over your curves better. If you're very angular or lack curves, wear crisp, tailored fabrics. This will showcase your body better.
Know what you're looking for.
Bring a checklist of what you want, so you avoid impulse shopping and buyer's remorse.
If you don't love it, don't buy it.
If you don't like it and purchase it anyway, it will forever hang in your closet as something you keep "just in case" you might use it one day.
Don't shop for the figure you wish you had; shop for the one you have now. Look at yourself in the mirror and try to determine your body shape. Are you heavier on the bottom? Broader on top? Look for items that flatter your best parts and mask your flaws. You can't just pull something off the rack and hope it's flattering. Really take your time and shop for your shape.
Don't pay attention to the label.
Try things on and shop for what looks best on you, not what the label says and what size you are supposed to be.
Don't seek the smallest size you can squeeze into.
A tight garment will expose all your rolls and imperfections. A perfectly fitting garment has enough ease to neatly fit without squeezing you -- no muffin tops, quad boob, etc. If you can't find the perfect size, err to the larger size and invest in alterations.
If you're thicker in the middle than you'd like…
Buy a top that fits closest at your ribcage and slightly looser over your belly. Try to avoid shirts with gathering at the top of the waist, though -- you don't want to be mistaken for being pregnant. Avoid extreme color contrast between top and bottom so you look longer.