The down:
Purpose:  the purpose of this exercise is to teach the dog to lie down.  The uses are the same as for the sit, but from a down position, it's a little harder to get up and going than in would be from a sit, and is a better choice for a more reliable "no-break" position.  This is also good for grooming, nail trimming and letting the vet get a good look at your dog.  

Action:  Have the dog sit first (praise, but don't click...remember the click ENDS the exercise).  Then take your treat and make an "L" shaped curve with it slowly, from the dog's nose to his feet and then parallel along the ground. You dog should follow the treat and stretch out on the ground to get the treat.  When the dog is completely down, click and treat.  You may have to do a little "shaping" with this exercise, especially if you have a dominant dog. The down is a vulnerable position for dogs, and takes quite a bit of trust to accomplish with more headstrong dogs.  Again, break the behavior up into little steps.  Click and treat for the dog dipping his head.  Then click and treat for him bending at the elbows. Then finally click and treat for him going completely down.  Typical cues for this behavior are "down" or "lie-down".  

Tips: An alternate method that I have found that works extremely well, especially for those "dominant" individuals is the elevated down.  Place the dog on an elevated surface with a ledge, such as the couch, a grooming table, a chair or for your larger dogs, the tailgate of a truck.  Take the treat and "guide" the dog in the same manner described above, except that now, you can actually get the treat in a negative relation to the dog (below the dog). Thus forcing them to lie down to lean over the edge of their perch and reach the treat without falling off.  When they are down, click and treat.  Eventually they will get the hang of it, and not have to be elevated any more, and can perform this on a level surface.