This Washington Post article detailing how Rev. Richard "Rich" Weaver crashed a presidential party and managed to hand the President a note before being escorted away demonstrates something I've always found to be true: if you look and act like you're supposed to be somewhere, people will generally assume you are. My own personal experiences:

• I once walked up to the front of a very long line and stood at the door without a word from the people in line. I was carrying a ton of camera gear around my neck and in a bag, and when asked merely said I was here to photograph the event (which was true, but it was only because a friend was in the contest).
• Amazing but true, with the advent of cell phones people will sometimes call for pizza delivery at a sporting event. In college I walked right through the back door of the stadium carrying nothing but a pizza box, wearing my dorky-assed red-suspender uniform. If I'd had street clothes in that box instead of a pizza, I could've changed in the rest room and watched the game.
• "Old Main", the first building on the U of A campus and a very impressive (for Arkansas anyway) piece of 2nd Empire architecture, had been undergoing renovation for nearly four years, and by 1991 was essentially complete. One day when walking by I noticed the security guard for the front door was standing thirty yards away hitting on a co-ed, so I simply squared my shoulders, put a serious look on my face, and walked right in without looking around. Once inside I had the run of the place, tinkered with lights and actually crawled around in the attic.

So if you really want to go somewhere, dress nicely, act nicely, assume that you're supposed to be there, and there's a darned good chance you'll go right in.

As long as you don't carry a gun!