First impressions do matter, especially when you are meeting your boyfriend's parents. Your skirt shouldn't be too short (no more than an inch or two above the knee), and your top shouldn't be cut too low. The last thing you need is his mom telling everyone, "She dresses like a tramp." By the same token, keep your makeup simple, not overdone.
Keep in mind that dressing appropriately doesn't always mean dressing to the nines. If you are meeting at a casual restaurant or his parents always wear t-shirts and jeans, it doesn't make sense to wear a suit or fancy cocktail dress. Overdressing will make you seem snobby, while dressing too conservatively may make you appear uptight.
Talk with your partner about what he thinks is appropriate; get some insight into his parents' style, too.
Generally speaking, a sweater, knee-length skirt and low heels will work if you are meeting his parents in their home. Otherwise, dress appropriately for the location where you are getting together.
If you are heading to his parents' house for dinner, bake something to bring. If you can't bake, enlist a friend to come over and help you out. Cake, cookies or any type of goodies will impress.
Whatever you do, don't buy something from the local bakery and try to pass it off as homemade. His mom will know.
Do your research.
What type of work does his father do? What are his mom's hobbies? Where did your boyfriend grow up? Do your research so that you can bring up interesting topics (or at least participate in a conversation) about the family instead of talking incessantly about yourself. Going on and on about yourself will come off as conceited, no matter how great you are.
It goes without saying that you should be polite to your boyfriend's parents, but go the extra mile. Greet them with a handshake and a smile. Address them by Mr. and Mrs. _______, not their first names (check first that they share your boyfriend's last name, so you won't need to be corrected). Be complimentary to your boyfriend throughout the visit.
After the meal, offer to help with the dishes. Don't stay too late at their home, and when you are leaving, mention how much you've enjoyed meeting them. The next day, send a "thank you" note (yes, in the mail) for having you at their home.
Make an offer for next time.
Though they might not take you up on it, offer to take his parents out to dinner next time, or offer to cook for them at your place. Be sure to let your beau know that you are going to do this beforehand. Asking to take his parents out to dinner — your treat — will let them know you are financially secure (and not after their son's inheritance), while also hinting that you are going to be sticking around for a while.
More on Meeting his Parents
Meeting his family: How to charm and what to avoid
8 Ways to make his family love you as much as he does
What to wear when meeting your boyfriend's parents