Be polite to the DJ. If you want to get on the karaoke song list, put yourself down for one track at a time. Don't demand to be heard 10 times. Know which karaoke song you want rather than asking for suggestions. He's busy and doesn't know what you like.
Do not scream into the microphone. Certain microphones are more sensitive to others. If you are too close to it, people might just here garbled nonsense. Don't tap on or swing the mic, either.
Do not exaggerate notes and act like a diva. Be realistic about your voice and singing ability. You might great at a number of things in your life but this probably is not one of them. If you were Whitney Houston or Billy Joel, would already have a record contract, not seeing at a karaoke night. Remember, you're there to have fun, not impress a music producer.
Share the mic when performing a duet. Keeping it to yourself kills the spirit and ruins the party.
Do not steal the mic from another performer when it is her time to sing. Even if you hear an artist or musician you love and are tempted to join in, do not do so unless you're invited by the singer. Even if you stay in the crowd, keep your singing along to a minimum -- don't try to out-sing the singer.
Applaud no matter how bad a singer is. Never heckle. After all, someone who's not a professional had the nerve to get up there and sing in front of everyone.