First Impressions Matter
As stylist Rachel Zoe once noted, your choice of clothing says who you are without you having to say anything. When meeting your boyfriend's family for the first time, it's best to err on the side of caution and wear something conservative yet feminine in a classic silhouette. (No matter how rockin' your bod is, clevage and short hemlines are definite no-nos for this occasion!) Your accessories are the best place to play up your personality, so long as your overall look says "classy, future daughter-in-law" and not "I am a phase your son is going through."
Mind Your P's and Q's
After your appearance, the next thing his family will be silently observing is your behavior. If you are feeling a little shaky about the rules of etiquette, like which is the salad fork (the one furthest away from the plate) or when to start eating (after everyone is served), it can't hurt to skim through an book to brush yourself up on the basics. Also, if you're eating dinner at their home, it's a nice gesture to bring a thank-you gift, such as fresh flowers or a bottle of wine, if they drink.
Think Through the Visit
The success of the dinner conversation you have with his family can make or break your visit. It's best to ask your boyfriend a lot of questions ahead of time and have him debrief you on any sensitive subjects or conversation topics to avoid. Likewise, try to find out a few of their interests and hobbies as well. For example, if his mother has an interest in Eastern philosophy, some clicks around Wikipedia pre-meeting will at least provide you with a few fallback questions to use as filler, if the conversation should run dry at any point. It also makes you look thoughtful that you cared enough about getting to know them to find out their interests.
Have Fun
Lastly, don't forget or be too nervous to enjoy yourself! Approach the meeting as an opportunity to get to know your boyfriend's family and become their friends. After all, you both love your boyfriend, so already you have something important in common.