You can't really rush them, it's a personal thing. My granddaughter was 3-1/2 before she finally decided it was time. It's not an easy concept to "understand" and I think helps if there can be conversation between parent and child. My granddaughter worked well with having a kitchen timer set and then asking her if she'd like to "try" at that time.

I like the Baby Center's articles: http://www.babycenter.com/0_the-abcs...aining_4399.bc