"Excuse me, sorry, erm... Do you know how much a polar bear weighs? Enough to break the ice. Hi I'm *insert name* would you like to see some magic?" ( I use that at parties, but I leave the magic bit out, remember you dont want people remembering you just for the magic, surely you want to be remember as a person to)
Or here's one a street performer told me tonight, this is how he starts drawing a crowd. He holds a piece of string between his fingers just so it above the ground and shakes it a bit and says "String dangling!". He says that randomly and eventually someone will stop and ask him what is going on and then he goes into his other crowd drawer.
Basicly, you just have to be yourself, what ever works for one person may not work for you. It all depends on who you are, you just have to go out and try different things out.
Cheers, Tom