DON'T SING BALLADS
I can't stress enough how important this is. Do not choose to sing a ballad. For one you probably won't be able to hit the high notes. And for another thing ballads take damn near forever. Thinking about November Rain, THINK AGAIN!
SING WITHIN YOUR RANGE
This goes to the point of you trying to hit a high note. You can't do it and you just look even stupider than you think you already look. Your falsetto is better left unheard. Trust me.
DON'T LET OTHERS PICK SONGS FOR YOU
In karaoke there is only you and music, if you don't know the words, you look like an idiot and everyone knows it. How does someone else know which songs you know by heart? Unless you live with them they probably don't. You may think you know chorus, and maybe you do, but how about the rest of the song? Of course the words are on the screen, but it's not as easy as just reading them, I've seen countless karaoke victims fall to the "but I know the chorus" excuse for choosing a disaster song like Rick Martin's "Ole Ole Ole".
MAKE OTHER PEOPLE SING WITH YOU
Mask your suckiness with your friend's suckiness. This makes singing karaoke more fun too. Be careful of duets, however.
DRINK ALCOHOL
Alcohol makes you sound better. Well, it makes you think you sound better. And if everyone is drinking, everyone sounds better. And if everyone gets trashed well then you won't remember who sucked the worst. You might even be so bold (read:drunk) as to hop into another karaoke room and hit on the cuties singing in there (I've done it before!). So drink up!
HAVE AT LEAST TWO STANDBY SONGS
You need to have to strong songs that you know by heart and that have strong group choruses. If everyone sings the chorus then you'll be fine, just know the rest of the damn words. My standbys are Oasis - Wonderwall and Creedance Clearwater Revival - Have You Ever Seen the Rain. Strong choruses and just about everyone, even Japanese people, know these songs. Try to find similar.
DON'T CHOOSE TOO MANY SONGS
Let the other people choose as much as they want. You'll sing less and be able to drink more. I can't see why someone would want to sing three songs back to back to back but I've seen it happen. Disaster it was. Keep better track of what songs you've already queued up to avoid this pitfall.
JOIN IN CHORUSES YOU KNOW
If you look like you haven't sung anything at all you're a target to sing next. Join in the choruses of others and you might be able to get by not singing anything of your own. Score! But if you're trying to impress a J-girl, you must sing at least one on your own if not more.
NO HIP-HOP, NO RAP
Now I like hip-hop--at the club; not when I'm singing in a small room in front of people. 99% of all hip-hop and 100% of rap does not go well in karaoke. Stick to pop or rock and you'll be golden.