Treat their daughter with respect.
The most important thing you can do in the presence of her parents is act like you care about their daughter. Considering this is your girlfriend we're talking about, this should be the easy part, right?
Pay attention to how you address her; "Sweetheart, can you pass the salad dressing?" vs. "Get me another soda" conveys more than you may think. Also, be aware of how you touch her. Holding her hand is a sweet gesture for a parent to observe, squeezing her ass is not.
In general, keep the affection PG. You want to show them you have a healthy relationship, not a healthy sex life.