take ur finger and gently press the small of their back down as u say it. do this for 10 minutes a few times a day if you can. Ryoko learned this in less than a week if i recall. I also did a head down command with him in the same fashion, when he was in the down position I gently pressed his head down while saying head down, and the next day he was doing it with just the command!
u can treat them as you see fit, I try not to treat after the first few times because Yorkies are smart turds and they will figure out that this is what i have to do for a treat, if she does not have a treat in her hand, if i don't see or smell a treat, then i ain't gonna do no tricks!.
Depending on the difficulty i use a treat(which is his own food in my case) until they get the idea. Then i do the command as often as i can with no treat and at least once a day.
http://www.loveyourdog.com/tricks2.html#kiss
combine what you think works best with the advice you get