Her parents will be assessing and evaluating every word you say to try to read your character. But turn the focus onto them, as well -- after all, the goal should be for you to get to know them, too. You'll want to show interest in her parents while controlling the conversation to shed a positive light on yourself.
Here's where your reconnaissance work comes in handy. Talk about shared interests, or at least ask questions to learn more about theirs. Unless your girlfriend has warned you that her parents hate their jobs, ask them about their work. Another great topic is their background and upbringing -- this shows you care about family history and traditions.
If you've learned that you disagree with the parents about politics or religion, you'll want to avoid these topics. Though such conversations are probably inevitable, the first meeting is not a good time for them. If these topics do come up, restrain yourself from arguing and politely guide the conversation back to pleasantries. And if you're intimate with a girlfriend whose parents hold more traditional values, avoid the topic of sex and everything remotely related to it.
Keep several neutral conversation topics in your back pocket to use when you need to refocus the conversation or avoid awkward silences.