Making a good first impression with his parents isn't all that different from leaving a positive impression with anyone you meet. While there are certain points to keep in mind that are unique to this situation, the most important thing to remember is to be yourself.
You want to give an accurate picture of who you are when meeting your potential in-laws. To make a good first impression with his parents, however, think about what kind of person most parents would want in a mate for their son.
Everyone wants to be around someone with whom they feel comfortable. A warm smile is the first way to let your guy's parents know that you are a kind, open, friendly person. Ask your honey a few unobtrusive questions about them beforehand. Being prepared will give you an idea of his parents' interests, making it easier to come up with topics of conversation in case you get nervous. Showing a genuine interest in them will make them more comfortable with you as well, and chatting for the first time will be much easier.
Make a good first impression with his parents by showing up with just the right gift. A bottle of wine is always nice, but make sure that your sweetie's mom and dad drink. Teetotalers will not be impressed by a gift of alcohol, and may, in fact, be a bit put off. If they do drink socially, find out if they care about wine, or if your present will just be relegated to a wine rack or the back of a pantry. If they are wine lovers, be sure to pick out a quality wine that you can afford.
A nice bakery treat, like a batch of cookies in a pretty box, will make a good first impression with his parents, provided neither of them is trying to lose weight. You should also be sure that neither of his parents has a medical condition like diabetes before bearing a sugar-laden gift. Being considerate is a good way to make sure his parents' perception of you is a positive one. After all, if you're thoughtful upon meeting his parents, they will be more likely to see you as someone who will think of their son's needs as well.
Another way to be a hit with his parents is to dress appropriately. Again, you shouldn't try to make a false first impression, but you should try to leave his parents with a positive perception of you. Dressing modestly is a good start; lots of cleavage won't exactly make a positive first impression with his mom or dad. Beyond that, don't worry too much about your clothes. As long as you look well-groomed and neat, your outfit is far less important in your quest to be a hit with his parents than the way that you treat them and their son.