Sitting is not the most natural position for a dog to be in. If you observe dogs or wolves in the wild, they are most often standing or lying down. Because of this, teaching your dog to lie down is a useful command, especially when you need your dog to hold the position for an extended amount of time. Your dog will be much more comfortable than if he or she was required to hold a sit stay for the same amount of time.
As I stated in my previous clicker post, there are various ways you can go about teaching your dog a new command. For a 'down', luring typically works the best. Most dogs naturally follow a bit of food with their eyes and nose. Therefore, by using a treat to lure your dog's face towards the ground, that alone is most often enough to achieve the proper result. As with any clicker training session, immediately click and reward a successful down. I never advocate forcing a dog into a position. However, if luring does not seem to be enough, you can also mold your dog by gently massaging his or her shoulder blades to help encourage them to lie down. The leash can also be a gentle guide to pull your dog in the right direction, if extra help is needed. It is important to keep in mind that a 'down' is the most submissive position for a dog to learn, so it is best not to start with this command until some other training has happened and the bond between dog and handler is established.
One problem that people frequently run into with luring is that they have trouble moving away from it. Meaning, their dog will not perform a command unless the lure is there. The key to getting rid of a lure is to do it gradually! So, if you have been holding the treat all the way near to the ground and your dog is consistently lying down, try holding it an inch or so above the ground instead. If your dog can still lie down consistently, hold the lure two or three inches above the ground. As your dog becomes consistent at each modified way of luring him/her, you can add more and more space between the lure and the ground. The ultimate goal is that, eventually, your dog will lay down on command, even when you are standing up straight.
Do not attempt to increase the duration of your dog's down stay for more than a moment until he or she is really grasping the concept of the word. If you try to step away from your dog too soon, they will surely get right up, and it is easy to become frustrated. Focus first on the simple concept of what a 'down' looks and feels like for your dog - and allow their muscle memory to kick in as well. You always, always want to set dogs up for success!
Luring Gabe at a beginner's level
Luring Gabe at an intermediate level.
Excuse my very unprofessional pictures - I had to use a self timer and also hope that Gabe's timing would be accurate!