Wait till their ready developmentally not chronologically. Typically demonstrated by an interest in the potty, dry diapers at night, and the ability to recognize when they're "going" or need to go. I took the sticker chart approach. The day he turned 3, I placed him in underwear, made a HUGE deal about being a big boy now, and explained every time he went in the potty he got a sticker. At first 5 stickers earned him a new matchbox car, something VERY important to him. As he got better it was 10 stickers to earn a prize, then eventually he lost stickers for accidents due to laziness, and gained stickers for going in the potty. He was "trained" without too much stress in 2 weeks for peeing, 4 weeks for pooping. He's now 5 and I don't believe we've ever had an accident, not one I can remember anyways. The key is waiting till they're ready, not you. And lots of positive reinforcement, no punishment.
at 8:26 PM on Mar. 30, 2011