It really helps at the beginning to remove the training wheel and have the bike seat low enough that their feet touch the ground.
We also took the pedals off the bike, took her over to the parking lot of the school (giant empty paved area with a very gentle slope), and let her coast while balancing and steering. Then added the pedals back in, with someone holding her while she pushed off. Then she tried pushing off herself. It took about an hour before she was completely independent and confident, with only one wipeout and no scabby knees.
We haven't kept up with it, though, so she's lost most of the bike-riding ability.
