TEACH
A good trick to pair with others (Back, Down, Roll over), and also a useful one. You can have your dog back out of the kitchen while you're eating, for example. It might be fun to think up creative ways to use Back with other tricks for a much more impressive show.
STEPS
Step One: With your dog standing facing you, walk forward toward him. When he takes a step backwards, click and treat.
Step Two: Continue stepping forward. When he takes multiple steps backwards, click and treat.
Step Three: Practice this until he understands that if he backs up, he deserves a treat. Start walking toward him less. You can use a signal instead (Try waving the back of your hand toward him, or taking a single step forwards).
Step Four: Once he has learned this, say "Back" as he backs up and as you give the hand signal. Do this several times until well learned. Remember to click and treat when he cooperates.
Step Five: Give him the command and see if he'll back up! Click and treat well each time he obeys. Reinforce with extra training sessions.
TEACHING TROUBLE
My dog won't start backing up unless I advance toward him.
That's where the signal comes in. Try using the signal as you walk toward your dog each time. Then, gradually stop walking forwards, but keep the hand signal. Be patient and give your dog time to think about it. Also, remember to take enough breaks and don't wear your dog out. Short sessions are the most successful!