This worked for both of my kids, but they were younger -- go to the top of a grassy hill (not too steep). Have them straddle the bike and then gently push downhill. Tell them to put their feet on the pedals, but not to actually pedal. Tell them not to put their feet down until they feel themselves beginning to tip once they have reached the bottom of the hill.
Repeat.
The momentum of the bike will keep it from falling over as it goes down, the grass will be soft, and they will learn that a bicycle can balance itself without too much input from them.
You don't have to chase behind them with bent back ...