Don't say I've only got time for a quick song and then sing a very long one

And if, for reasons best known to himself, the MC says something about "one each or two very short ones", don't take it as a green light for you to do two long ones, however well you think they go together. Some people may be bowled over by the sheer brilliance of your performance, but - those few people apart - everyone waiting to go on will hate you, and everyone who came to listen will think you're an arrogant dick.

(Seen at a poetry night. They weren't even good poems.)