To the stand up comedian in the grocery store check-out aisle:
You may or may not remember me, but I was in your aisle yesterday, and have some advice for your little one-man show:
Brush your teeth. If you're going to trap us in the check out isle and share your witticisms, please brush, or at least pop in a tic-tac after that cigarette.
Keep it short. We're trapped against our will. Our groceries are being rung up, and we're trying to pay for them as fast as we can. We can't leave. You win - so have mercy on us.
Get some new material. That stuff you said about Bill Clinton's been said before, and it wasn't really that funny the first time I heard it.
Don't steal the show. Interact, and let others participate a little. You aren't the only person in line who thinks they're funny.
Know when to quit. If someone has paid for their groceries, and are walking away from the isle towards the door pushing a heavy cart while trying to keep track of 5 kids, let them go. Don't keep trying to get in that last one-liner.