The Down Command is the second most important thing to teach your dog after training him to sit.  
Teaching your dog the down command may take a couple of weeks of practice because it is a little more difficult as it requires him to contain his energy and concentrate, but it is definitely worth the effort.  Once he masters it, you can use the down command to give him a break during walks or playtime, and it can help stop problem behaviors like barking as it is difficult for a dog to bark when they are lying down.
 
1.      With you dog sitting in front of you, hold a treat in front of his nose.
2.      Tell him ‘down’ as you simultaneously lower the treat to the ground at a slight diagonal away from       him, or a kind of down and out motion.  As his nose follows the treat, he will automatically end up in the down position. 
3.      Once he is lying down, give him the treat and praise him.

Disclaimer
Please consult the services of a Professional Dog Trainer, Behaviorist or Veterinarian before implementing any of the advice contained on this site.