-Be open to new styles and colours. Try on things you would never normally select.
-Don't prejudge an item before you have tried it on; most clothes do not look good on coathangers.
-Try on a whole outfit, even if you intend to buy only one item. Make sure you include accessories and shoes if possible. It will give you a much better feel for how it will look.
-Take your socks off. How can you fully appreciate the gorgeousness of a summer dress when you have on your dark socks with it!
Same goes with dark pantyhose and boots.