Take a look at yourself in the mirror in your regular clothing before you undress. Do the mirrors or the lighting seem especially flattering, or just awful? This way, you'll have some means of comparison when you try on store items.
Wear tops, bottoms, and shoes you can get in and out of easily. Opt for a skirt instead of pants, a pullover instead of a button-down, ballet flats rather than sneakers.
Choose the right undergarments. Don't go dress shopping in a sports bra. A convertible bra will work with various necklines, and you'll be able to get an idea of how the dress (or a blouse) will look. Similarly, if you're shopping for a swimsuit, wear a thong or bikini briefs as opposed to boy shorts.
Use clear deodorant and wear minimal makeup. When pulling shirts on and off, you don't want to worry about ruining any merchandise.
Keep jewelry to a minimum. Necklaces and large earrings get in the way when you're trying on tops.