here's what i've done: get small pieces of treats. have the dog standing. put your arm under their belly just in front of their back legs. put the treat in front of their nose, when they are interested, lower the treat to the floor so their front half goes down. you're arm under their belly will keep their back legs up. when the front half is down, give lots of praise and the treat. after they get the idea, add a command word, "bow", or "please" or "food" or "scrub the floor" or whatever. Once they've learned it, start making the treats random. they won't know if a treat is coming and it helps keep them guessing and obeying.