Well, rule 1 perhaps should be "don't spend more than you can afford," but I'm going to start with a rule that occurred to me this morning:
1. Shop for specific items. Don't go browsing; it's ideal to go online with a specific item and a price range in mind already. Browsing through stuff is an easy way to end up with things you don't need and won't use.
It's best if you have a specific product in mind too. For example, go shopping for the Manolo Blahnik Kidskin Low-Heel Halter, Navy in size 8 (I wish! Gorgeous shoe).
I was looking for decants of Bergamote by The Different Company, and ended up on Beauty Habit's Outlet page, a very dangerous page with Jurlique products and other delightful skin care and cosmetics offerings. I collected a cart full of products, and had to run off to work. Later on I'll cull out the unnecessary items and perhaps buy something.