One of the biggest mistakes you can make in rushing a fraternity chapter is pretending to be someone you're not. Remember that if you are accepted, you could be spending the next four years living and socializing with fraternity members. If you don't feel like you fit in with a fraternity's lifestyle, chances are you won't have a good time once you are accepted. Show the members who you really are, and they probably will be more impressed by your genuine candor more than anything else. If they reject you, you probably weren't a good fit anyway.
Fraternities want members who genuinely have an interest in their organization. Show that you care about the fraternity by asking a lot of questions to various members. Asking questions also shows that you are assertive, and can demonstrate the fact that you've done your research into their history. In the end, the answers to your questions can help you decide whether a particular fraternity meets your own standards.
While pledging a fraternity is a less formal affair than a job interview, it helps to show common courtesy to the fraternity's leaders. Do your best to remember everyone's name and address members directly. When you go out with your prospective brothers, offer to pay for food items. Even if they refuse, it shows politeness. Fraternity brothers may not have a reputation for using clean language exclusively, but take your cue from what you hear around you. If you observe only polite language, watch your tongue and avoid cursing.
In reality, many fraternity houses are not the 24-hour party venue that you see in movies. Nonetheless, alcohol is likely to be present at fraternity rush parties, even if there are underage attendees. If you do drink, drink responsibly. There is no better way to make a fool of yourself and ruin your chances at membership than by getting drunk and doing something stupid. Be moderate, be smart, be safe and you will leave a lasting impression on your prospective brothers.