Step 1: Blend in with the crowd, but not too much. You don't want to be mistaken for a real guest.
Step 2: Grab a glass of punch, but not the good stuff. You want to appear interested, not intoxicated.
Step 3: Make friends with the bartender. They're the ones who can really make your evening.
Remember, a good crasher knows when to make an exit. Crasher Etiquette: The Art of Leaving with Dignity