Do Your Homework
Don't go into battle unprepared -- do a little reconnaissance. The first essential step is to talk to your girlfriend ahead of time about her parents.
Get specific. What do they do for a living? What are their hobbies and interests? Most importantly, ask about their core values: What are their politics? How religious are they? These are potentially perilous subjects for you to broach if you disagree with them. Ask her if there are any other subjects you shouldn't bring up.
If you don't want to bombard your girlfriend with such questions, or if she gives only vague answers, ask her to tell some stories about growing up with them. This will give you an idea of what kind of parenting style they had. If they were strict, you know to walk on eggshells and show respect. If they were more free-spirited, you can relax a bit -- but not too much until you know them better.
Aside from preparing you for what to expect, these will be great conversation topics, which we'll talk about more later.