Befriend fraternity members. If they know you and the party is open to outsiders, you stand a chance of making the list.
Know the theme of the evening. If it's a formal, dress appropriately. If it's a costume party, arrive in the most outlandish costume imaginable.
Walk like you have a purpose and know where you're going. You don't have time for interrogation.
Come bearing gifts, whether they be snack food or the beverage of choice.
Bring women with you. It's a sure bet you'll get in.
Dance and party like a maniac. Maybe they'll ask you to rush next time.
Sneak in if you must, but always have an exit plan so you can avoid unwanted pummeling.