The best trick I've found is to get kids to try things that, to them, seem scary and impossible, but which I know that no one can actually fail at. Example: stand on the side of the deep end with them, jump in holding their hand, and push them to the surface as soon as my feet hit the bottom. They're not underwater long enough to panic, and then I'm holding them up in the deep end, and I can praise them warmly for jumping into the deep end...and even if they didn't jump entirely voluntarily, they can't exactly say 'no I didn't jump'.