Do Your Homework

I know it's all too easy to impulse buy, especially with beauty products. But you really will be much more likely to actually use said product in the event that you research it. This is what I've been doing since even before I started wearing makeup. There are books, of course, but they tend to become outdated quite quickly. There are two places that I'd recommend over anywhere else.

For a consumer opinion with a community feeling: MakeUpAlley. I've used their site for basically forever and there are a ridiculous amount of reviews on there. You can even sort the reviews by most popular, highest percentage of buy-agains, top picks, etc.. Not to mention that you can get real-time feedback on whatever it is that you are considering with their message boards.

For a science-minded opinion: Paula Begoun. She has a handful of reviews on her website, but to see all her reviews you do have to pay. You can either get a copy of her Don't Go to the Cosmetics Counter Without Me (usually at the library, although perhaps an older edition), or for around the same price get a subscription for one year to her Beautypedia which is constantly updated with new products. Paula definitely knows what ingredients work and has rigid standards for what you should waste your money on and what not. Her reviews have been quite biased in the past (her older books declare a profuse hatred for products with shimmer) but she is getting better at a broader opinion. I'll be honest here, she stays up to date (check out her blog) with skin-care research and to me, with skin-care at least, her word is law.

Bonus search: Google the name of the product you are looking for, and try to find blogger's reviews. Because of the time and care that most people put into their blogs you will typically find a much more thorough review, often after trying for a few weeks, and you will be able to see why the product worked for them because you can understand their skin background.