Some estimate that we express and receive 90 percent of communication not through words, but through body language [source: Reiman]. Whatever the exact percentage is, no one can deny that body language speaks volumes, especially during first impressions. Dr. Jim Houran, a psychologist specializing in relationships and dating, recommends using strong body language when meeting the girlfriend's parents. This means smiling, having an open, inviting posture (don't cross your arms), and maintaining a lot of eye contact [source: Houran].
But your actions don't have to be so subtle. Bring a gift -- a peace offering of sorts to lighten any potential tension. Wine and chocolates or flowers are a good stand-by. But get your girlfriend's approval to make sure they drink alcohol, and to check what kind of chocolates or flowers to get.
Overall, experts recommend being respectful to parents, without going so far as to cower in their presence. Likewise, offer compliments without being sycophantic. Most importantly, this will all prove to your girlfriend that you care.