Do your homework in advance. Every girl is different, so every girls' parents are different, with unique expectations. All of the recommendations that follow can backfire horribly if you don't consult with your girlfriend first about what her parents are like.
Find out as much as you can about the parents from your partner. What do they do for a living? Do you have any common interests? Have they been on holiday recently? Are there any subjects which are off limits? This gives you a heads up on conversation topics - and what to avoid.
Dress to impress. Going to meet someone's parents in your vintage, hole-ridden, acid-washed jeans and Che Guevara t-shirt that you got from the thrift store the other day for three dollars probably won't get you their utmost respect--unless they're very unconventional parents. This doesn't mean wear a suit - just be presentable (e.g. a polo shirt or t-shirt that doesn't spread the Communist agenda). Dress smart but casual, and make sure everything is clean, decently fitted (nothing too tight or baggy) and tidy. Get a fresh haircut and trim your nails.
Arrive on time. Don't pull up with your stereo blasting heavy metal.
Be confident when you meet them. Give the father a firm handshake.
Make good conversation. One of the most pivotal parts of meeting anyone - parents, girls, the president - is the conversation.
Treat your girl well. Be extra attentive to