|Teaching your dog Down|
Teaching the down is quite straight forward but requires some patience. Get some treats and find a quiet time to practice. Some larger dogs like the Great Dane may find getting into and out of the down position strenuous, avoid doing too many repetitions with very large breed dogs.
Place your dog in a sit and hold the treat in front of his nose, but don't give it to him. Now move the treat down slowly, as if you were going to place it between his front legs. Your dog should be following the treat with his nose, and as your hand gets lower to the floor he should lower himself into a down position. As soon as your dog is down, treat and praise.
Repeat this process a few times, and when your dog seems to be getting the idea start adding the word 'Down' as he is in the act of lying down. It is very useful to add in a hand signal too at this point - move your open hand, palm down, in a downwards motion as you say the word 'Down'.
Continue to repeat this exercise, coupling the word Down and the hand gesture with the lowering of the treat and the dogs movement of lying down, and treating when the dog is in the down position. After 10 or so repetitions try asking your dog to 'Down' with the word and the hand gesture alone. If he does it - Great! Treat and try again. If he does not, just go back a step and keep coupling the word with the action.
Proofing the Down
Once your dog has the idea you can start to make the Down a regular part of your dog's day just like the sit has become. Ask him to Down before a meal, or for a treat, or for a game. Ask him to down in different places and situations, and with different distractions around. When your dog is becoming good at the down start giving treats randomly, or only for the best downs, so that your dog is not expecting a treat for every down that he does, but is hopeful of getting one anyway so he will continue to comply.