Keep a little bike with training wheels on it after your child has outgrown it.
Get a balance buddy.
Here's how I know it works.
My son will turn five next month and he has been riding a two wheel bike since the beginning of school. At the time he was four years and three months old. (I add the months because he was just over four instead of closer to five.)
On the first day of school my son and I were getting ready to walk to the bus stop to get my daughter. He said he wanted to ride the two wheel bike. (I had already taken the training wheels off the little bike at the beginning of the summer, thinking my daughter was ready. She crashed once and was not willing to crash again, so she refused to ride without training wheels.) I found this nifty piece of equipment to help in two wheel training.
It's called the balance buddy. It is so helpful because you don't have to bend over and hold the seat and handle bars of the bike. Instead you can stand tall and still help your child stay balanced. I was able to run upright alongside my little biker instead of awkwardly bending over.
In addition to the balance buddy, my kids started riding a two wheel bike on the small princess bike my daughter got when she was three. They completely outgrew it but when it was time to ride without training wheels I had them use this bike.
It was easier for them to start the bike rolling if they were able to have both feet on the ground. Also, they could easily put their feet down when they started losing balance. It worked great.
By the time my daughter got off the bus, he was able to ride as long as I could help him get started (again, the balance buddy was super useful.) It didn't take long for him to figure out how to start on his own.
Not to be outdone by her younger brother, my daughter wanted to try. She too was able to ride the two wheel bike without much assistance that first day.
I do think part of it was that they were ready. A big part, though, was having the right equipment. A little bike and the balance buddy.
I'm liking up to Works for Me Wednesday and The Diaper Diaries for Things I Love Thursday.