What you wear will certainly influence the impression that you make on your boyfriend's parents when you first meet them. Showing up in ragged jeans, an old T-shirt and flip-flops probably won't do the trick. Neither will wearing a supershort mini skirt or a cleavage-bearing top. Your outfit should show that you respect them. The best thing to do is to dress on the conservative side -- think about what your parents (or even grandparents) might like to see you wear. Keep your jewelry small and tasteful and your makeup and hair understated. Don't dress as if you're going on a job interview; it's important to still express your personality. Just tone it down.
Your outfit should also be appropriate for the occasion. You'd look ridiculous wearing the same outfit to a backyard pool party that you'd wear to an expensive restaurant, but you can still look your best no matter what. As you get to know your boyfriend's parents better, you can always relax a bit when it comes to wardrobe, but you might not be able to recover from a serious fashion faux pas.