When it was time to teach my kids to ride bicycles, I first started with the traditional method of holding the back of the bike while running along behind them. That did not work well for either my children or my knees. In the search for a better way, I landed on using the method found at PedalMagic.com.
This site sells you a relatively short video to watch online. The video demonstrates a non-intuitive but effective method for children to become acclimated to balancing on a moving bicycle. In my own experience with my non-athletically-gifted children they all learned to ride using this method in 10-minutes or less. I have since helped other neighborhood children learn in a similar amount of time.
Arguably, a method such as this might be considered more of an intangible “hack” than a tangible “tool” – but for me it was very cool either way.
[Given the cost and nature of the training, Pedal Magic offers a straight forward guarantee if you're not satisfied with the video. --OH]