I'd suggest for a full grown dog, sitting first. After dog is seated, praise then command calmly "dooown..." while pulling both front paws up off the ground and out in front of dog extending the paws... ease pup's belly onto ground.... praise.
It will come. After a few sessions, you can do your obedience lessons on lead. Stop walking pup, command sit...pull paws out and command down. After some success there, transition to stopping in front of pup, command sit, then slide lead under your foot...pull lead taught and step down on lead while commanding down. apply pressure as needed between shoulder blades.
Take it slowly and in baby steps...it will come. Be consistent. Praise for good effort and for the dog showing success.
I've also seen some pretty awesome work done with ....GULP.... Clicker training to teach a dog down.
http://www.youtube.com/watch?v=Ciy7uUtA6S8
I'm not huge on clicker training, but it works...it really does. And if you're wondering how to get a full grown dog to do it, and the dog has brains...what have you got to lose?!
Chris