The only way we've been able to get Brody to do "down" is by sitting on the floor. Then we place one leg into an arch, foot flat on the ground and knee bent. We have to slide our foot out a bit to get it low enough so that Brody can't just walk under it. We place a treat on one side out of his reach, but close enough for him to see. Brody then goes into the "down" position to try and crawl/wiggle under our leg to get to the treat. Once he's down, we praise him for "Down" and give him the treat.
It took about 2-3 days before we could just put the treat in front of him and say down and have him do it.
I have to credit this idea to one of those trainer shows. I think it was It's Me or the Dog. I'm amazed how well it worked for us.