I know this is kind of old, but I thought I'd throw my 2 cents in! I am teaching my horse to bow right now, I actually posted a video of her doing it in the video section.
This is how I taught her to do it...it's worked so far. Not quite sure what everyone else does? I did it in 3 sections
I first started out with carrot stretches between her legs, to show her what I wanted her to do with her head.
Then I had her lift her leg while I stood at her middle, and with the lead rope coming to me from around the opposite side I would shift backwards pulling her leg slightly, pulling on the lead as well. When she would shift backwards, I'd release and praise.
Then I combined the two and we did it!
Depending on the horse, you could probably get them to do it on the first day. I'd suggest you switch off with treats and just praise, so they don't rely on just getting a treat. Treat one time, praise the next. So on.
Hopefully this helps!