Your dog should be trained to lie down immediately with its front legs extended in front of it. It can, if you desire, be a natural progression from the command to "sit!" or can be used independently. Give the authoritative command "stay!" as you place your hand on your dog's shoulders and apply gentle, yet firm pressure downward.
You then should stand in front of the dog and praise it for a job well done.
Allow the dog to get up after a short while, and repeat the command and hand pressure until the dog obeys without hesitation.