I've always just lured it - start with the dog in a sit, hold a treat just barely above his nose, and lure it backwards toward his tail until even one foot comes off the ground. Click the foot coming up. Continue this way until he's lifting both feet, then holding the position for longer and longer.
It's physically a difficult behavior for a lot of dogs, because it requires them to use a lot of muscles they don't normally use. So take it slow in the beginning and practice it often to keep his conditioning up.