In addition to rewarding a spontaneous Down, I taught Elka Down on command by luring. I've seen many methods described, some that I didn't really understand the machinations of at all. The way I thought was simplest was to have Elka in a Sit, let her sniff a treat in my hand, and then lower it to the floor. The nose is supposed to follow the treat to the floor, and in doing so, the paws extend so the elbows are against the floor, and presto! The dog is sitting, and treats and praise are lavishly administered. After several repetitions of this (depending on the dog, and the treats, your results may vary).
For a hand signal, I again more or less freestyled my own. For "Down", I hold out my hand so it is parallel to the floor (or ground or sidewalk). To implement it, I used the signal while also using the voiced command. Now Elka will respond to one without the other; should it be necessary to Down her while on the phone, for instance, it can be done with just the gesture.
There is also the difference between the "obedience" Down pictured above, which can also be considered a "Sphynx position" (as I've seen referenced in many dog books), and a more relaxed "go lay down". There are times you need your dog not to be underfoot, and this is a useful thing for her to respond to.
(notice the comforting Gumby presence?)