Step 1: Choose a command. I use two gentle taps on the top of the foot.
Step 2: With your dog in the sitting position, give the command. Using a treat, lure your dog into the down position by slowly bringing the treat down to the floor. Make sure to bring the treat away from the dog so that he has to stretch forward to reach it. When his elbows hit the floor, release the treat to him.
If your dog is having trouble getting in to position, break the behavior down in to smaller steps but lessoning the criteria and rewarding the dog when his front feet move forward even slightly, until he understands what he needs to do to be rewarded.
Step 3: You can begin to fade the lure by giving the command, and waiting a few seconds to see if he offers the position by himself. If he does, reward heavily. If he doesn't, he is not understanding what is being asked of him quite yet, so move back to the previous step.
Remember to keep these sessions short - even just a few minutes at a time, to keep things interesting for the dog. Always end sessions on a high note, and set your dog up to succeed. If you find yourself, or your dog getting frustrated, take a break and try again later.
While these methods have worked for us, it is important to remember that every dog is different and what works for one, may not work for another.