If someone comes in after the performance has started, stop what you're doing and say, "You're late! Ok, here's what you've missed" (goes through major points of performance so far, such as song titles, etc.). Or, if they have to step over people to get to their seats, it's funny if you sort of mumble "'scuse me, pardon me, sorry, 'scuse me" until they sit down. (Got that one from watching the great classical pianist and humorist Victor Borge.)