h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

You need to start with a bike with removable training wheels. These should be set at a high angle so that the child doesn't get into the habit of just leaning on one side. Allow them to cycle round merrily for a good few weeks to get them used to the pedalling side of things and how the steering workd etc. It may seem obvious now - but even this is a cause and effect system that has to be learnt.
Stage 2 involves removing the training wheels. For this you will need to go to a large reasonably flat piece of grass - probably the local park. (If your garden is large enough to learn to cycle in you are a rich so and so and should probably get one of your butlers to teach the offspring to cycle )
Place heavily padded child on the bike and hold it BY THE SADDLE from behind and get the child to pedal along while you provide stability. As they get up some speed you can release the saddle and they will be riding unaided. You should still run along behind them - a rapidly receding voice is a dead giveaway that you aren't holding on to the bike. If the child does slow down and fall off the bike, perform the most spectacular theatrical dive you can imagine...this distracts the child and they forget to worry about themselves being hurt...which on a lawn and padded up they shouldn't be anyway.

Of course as soon as you have them riding a bike you need to instill a lot of road safety in them...simply banning them from the road aint going to work. But that's another topic....

Or a small gentle gradient, where steering and pedalling are not so important, and just keep going down it, to get the balance sorted. Once that is sorted peddalling and steering (which came from practice with stabalisers on) can be fitted in naturally. Well that was the way I learned (I think - it was a long time ago) Until later....
BCNU - Crescent

Teach them to look at where they want to go, not what they want to avoid hitting. I've seen so many people (children and adults) ride into trees, rocks, walls, even off bridges, because they were staring fixedly at the object they were trying to avoid.

People who walk into walls enrich all our lives .

Guilty as charged...

Also, falling upstairs is a particular favourite of mine.

Actually, my 5 year old niece pretty much learned all by herself. One day, we both decided that I would teach her. Somehow, with me hanging on behind, she just couldn't get the concept of 'if the bike starts to tip *this* way, lean a little *that* way'. She kept leaning *this* way, which would start to pull auntie over... (No, my balance isn't that bad, but I was bent nearly double holding on.) I kept saying that, if I only had my bike, she could watch me, and maybe that would help. As it turned out, someone came along, riding a bike. Less than ten minutes later, my niece was merrily narrowly avoiding parked cars What really made it impressive is that I had been banished to the front porch, so she actually did do it herself!

I haven't tried this but I have seen it suggested that the best way to teach a child to cycle is to remove the pedals and let them use it like a velocipede. This teaches them balance first. Once they have the hang of it, put the pedals back on and Bob should be your uncle.

Papercut, you're exactly right. This is how I taught my oldest to ride (the youngest doesn't really have interest in anything with wheels). He was able to ride without training wheels and stand while pedaling by the age of three. Training wheels impede the ability to turn correctly. Best to never get a child used to them.

The next step is to actually buy the right child's bike after that. Many children's bikes come with that annoying coaster brake. That doesn't allow children (or adults for that matter) to get their footing to enable them to continue pedaling. Why? Because when you backpedal at all, the brake locks! As very few companies make kids bikes without coasters, you may have to do like I did, and custom order the back wheel/axle.