I will second the suggestion for a balance bike (bought or created). We took the pedals off both our kids' bikes and let them use them as balance bikes for as long as they wanted. We didn't bother taking off the chains. We found the best place to bike was at the local elementary school, which has a large area of blacktop that is slightly sloped. We also gave frequent rewards (usually jellybeans) for completing small goals.
The training wheels did not seem to help, and it wasn't until we took the pedals off that we started to see progress.
It took both kids over a year to learn, and DD7 is still not confident enough to bike anywhere but the school blacktop. Definitely worth the patience!
