Play dead
Your dog needs to know down in order to teach this. Starting with your dog in a laying down position:
-Slowly move a treat past his head so he turns to look at it and follows it with his eyes.
-As your hand (with treat) gets near the back of his head, move it towards his body
-If the dog is following the treat with his eyes and wants to eat it, he should naturally roll on to his back.
-As soon as he is in the "dead" position, TREAT!