Use Paraben-Free Cosmetics
Parabens are a preservative that cosmetics companies have used for years. Lately, one study linked them to breast cancer and another study linked them to skin aging. I'm no scientist but I wouldn't be surprised if those who are (like my dad) say that two studies aren't enough to conclusively say that we shouldn't use parabens. Still, why the hell would you put something on your skin and in your hair that wasn't conclusively tested for safety??? They can keep studying parabens and in the meantime, I'll avoid them.
You can tell if your cosmetics include parabens by checking the ingredients for ethylparaben and methylparaben, the most commonly used combination. Alternatives to parabens *DO* exist although some companies (such as Avon and Estee Lauder, who owns brands like MAC and Clinique) refuse to switch.
I suggest checking the Skin Deep report if you are looking for paraben-free products. I've had the most luck with Aubrey Organics on this issue and others. Like other brands that have paid attention to this issue, they market themselves as "Paraben Free." Which paraben free brands and products have worked for you?