OK, here's my method. It worked on my 2 boys (who were having the darndest time getting it) as well as a couple of neihbor kids. Now I tell everyone:
Hold the kid, not the bike!
I was initially holding the back of the seat and the handlebars and letting go. The kids never got the balance and then crashed, over and over again. Scrapes. Tears. Stomping. Frustration. And that was just me!
Then I went to the grass and held right under their armpits and ran along side, releasing my hands - first for 2 seconds, then 3, then 10 -- from their body but keeping my hands in place to grab them quickly when they were about to fall. It worked within about 15 minutes and this was after weeks of trying other methods.