Lay Down

Getting your dog to lay down on command is a lot like getting him to sit, it is done with food and the dog's ability to move in certain ways.

Get your dog to sit.
Take your hand with a piece of food in it to your dogs nose, once you get close to his mouth and he thinks he has the treat - slowly lower your hand with the food in it to the floor. He has no choice but to follow your hand to get the food, he also has almost no choice but to lay down to get the food.
If the dog does only a half lay down you can slowly move to food forward, once it has reached the floor.
    If the dog starts to stand up to follow the food, you might try having him go under a chair or anything to keep him from standing up after the command to lay down is given and before the treat is offered.
One important thing to remember is that once you issue the command to lay down and the food or hand signal is given, don't give the food or raise your hand until the dog does the desired task.