Down is the natural second command you teach your dog. I picked out a good video that demonstrates teaching down to a yorkshire terrier. She uses the lure of cheese to coax the dog to follow her to the floor. I like the way she goes from sitting to crouching down because sometimes a dog will listen to you when your standing but will not respond to the command if your sitting. She also has the dog do what I've heard called puppy push ups or going from sit to down to sit etc.
I've found it more difficult to teach our chihuahua to lay down because she is already so low to the ground. I had to use the technique where you sit on the floor with your legs out and lure the dog to crawl under your legs making their belly touch the floor. I've included a video showing the technique I used.