Info Note: I have a lifetime membership to a karaoke club
1) If you are in the audience, just clap. If they stank, who cares? It's about having fun... don't harsh someone's day
2) Don't do a song someone just did... it is kinda rude, and really boring. Everyone complains about the radio overplaying stuff, why should you?
3) "Don't Stop Believing" is pretty much always going to be a sing-a-long song. Please don't get huffy if the entire bar/venue starts singing with you and decide to stomp off stage. You just look like a jerk, and a good citizen will jump on stage and save the day, making you look even more like a jerk.
4) If the singing is so bad you can't take it, you can do something else. Son't be a jerk to the person on stage.
5) Please, for the sake of everyone, don't rub the mike on your sweaty groinal region while making fun of an 80's hari band. Yes, it is funny... but my lips have to go near that thing!
6) Don't point you gun at me for singing the song you like before you put it in as a request. I have a gun too... we all do. Moron
(#6 took place in Afghanistan. Unloaded weapon, but seriously? Bloodhound Gang had more than one song, and we had karaoke every week)