Hi! I've said it before and I'll say it again - I LOVE CLICKER TRAINING!
Check out
http://www.clickertraining.com
Treats work really well. The treat doesn't have to be big it just has to be enough for a taste - which is good especially if you're doing a lot of treating. I cut treats into pieces about the size of my little finger nail.
You can use the treat to lure the behaviour you want. For example for "down" get the dog to sit, then bring the treat down in front of the dog's nose and away from the dog along the floor. Once the dog is down give the treat and give lots and lots of praise. Once your dog's consistently going down start saying the word "down", or whatever word you want to use, AS the dog goes into the position you want so the dog associates the physical movement with the word. Then eventually you'll just be able to say "down" and the dog will go down.