1) Some senator or something did this once...
Take a couple of SCUBA tanks, place on bottom of body of water, preferably ocean (not too deep!)
Take your friends out on a boat, do some partying, drinking, act a lot more foolish than you feel.
"Accidentally" fall over.
Swim to bottom, get SCUBA gear, swim away.
Have some stuff conveniently stashed where you come ashore.
They never find the body, but very few people doubt you're dead.
2) You need an accomplice for this one, and you might have to wait too long to do it.
Wait for a large scale disaster... ferry sinking, massive building fire, tornado, something like that where there's no way to tell how many people died and you can't find all the bodies
Have your friend claim you were there.
a variation is to cause your own blown-up building, crashed bus, whatever
3) Join Army. Go to Iraq. Wait until an engagement, and disappear during it. All the better if some stuff gets blown up.
4) Make friend with someone on a hospital staff. Have them admit you, forge your chart, and have you "die." Walk out of the morgue. Slip the dead-guy-examiner a Benjamin on the way out to keep his mouth shut.