All of the advice above is sound, but for slightly different circumstances.
For very slow (walking speed) tight U turns, Jefros original advice is spot on. Increasing the engine revs is essential (it helps stabilize the bike), and feathering throttle and rear brake ensures a controlled stable turn.
For anything other than the above, get set-up with the correct road position / speed / gear, prior to the turn, then mildly accelerate round the turn, increasing acceleration as you exit. Again, this is the best way to keep the bike stable. Countersteering always being used in the turn, with counterbalancing as an option (speed dependant). Light usage of the rear brake on the turn (to scrub off some speed) can safely be used, if you do get in trouble (road position and surface conditions dictate how much you can get away with).
Looking where you want to go, rather than what you want to avoid, applies in all circumstances.
That's what Her Majestys Police Dispatch riders drilled into me over numerous training sessions (during advance rider training courses). What those guys can do on our roads over here (UK) is incredible to watch.
Anyway, for me correct road positon and always looking where you want to go, can cover a multituse of sins.
Hope it helps some way - Cheers and good practicing.