I borrowed a 2nd bike from a friend so my son wouldn't have to see the training wheels come off of his bike. It was a little bike that he could just use his feet to "scoot" around on. The guy at my LBS taught me this - he said they use this technique for teaching adults how to ride, too. So, they scoot around using their feet on the bike and you can put them somewhere where there's a gradual slope and they can scoot down it and they gain confidence very quickly. Remember though that balance is developmental and some kids don't achieve it for awhile. My son was 6 before we even tried it (he's 10 and still wobbly but he LOVES to ride), but he caught on in a couple of days.

Good luck! My 3 year old just got her first 2 wheeler for Easter - part of the next TE generation.