Tips on the wrists: make sure your wrist slides by the end of the grip. If your hand is on the inside of the grip too much, itll hit your wrist. This does not mean start on the outside, it just means you need to loosely slide your hand to the outside when doing the trick. Keep your hands where you always have them, there's no need for a special hand position. Bartwists come out good if you let them flow easy. LOOSELY (extremely important, holding it too tight means too much friction for it to flow, hence some people suggest water to learn to slide your hand properly) twist the grip, and slide to the end of the grip and come back.
Since yours bars are 18 wide, you'll want to lean your body back a little bit so they don't hit you.
Staying on the deck is commitment. Force your feet to stay on, learn em flat to ensure that. It's an awkward motion and it always dips the deck, one of the few tricks that does that and instinctively makes you want to come off.
Here's a vid of me doing some a few years ago right after I first learned them. If you pause at 10 seconds, you'll see my deck is almost vertical when I'm doing the motion. It's just dedication, man. Take the tips you know and PRACTICE. Once you know the technique, all there is is practice. There is no magic technique to learn something immediately. (
http://www.youtube.com/watch?v=lbJijzrfRxo )