I can tell you how I trained it - it was so simple. I had tried everything, gotten nowhere. Someone on the dogwise forum suggested this, as this is how she trained her dog. Sounded too simple, but wth? As dogs will almost always stretch when getting up after naps, etc, keep treats in your pocket, and stashed everywhere and capture that "bow" (stretch) and praise with an obvious YES and toss a treat between their two front paws when they're 'in position'. After about a week Makena was bowing all the time and catching my eye just to see if she could get a "YES" and a treat (and I was obliging) , that's when I added "queen" (as in ... hail or bow to the...) as the cue, and I also kind of curtsey/bow - so I now ask her to "bow to the queen". hehe But she caught on really quickly, it was almost too simple. Hope it helps.
One suggestion - 'bow' sounds a lot like 'down' and can be confusing to a dog, just because of the sound, so may be best to find a word besides 'bow' when looking for a cue.