Begin with your dog with a sit.  Slowly place your arm under the front legs of the dog, holding a treat above and in front of the dog's head.  As you raise the dog's front legs, repeat the desired command.  Slowly drag out the command as you raise the dog.  As soon as it is in the correct position, quickly treat it.  Begin again and eventually stop assisting it.  Encourage him/her to raise itself only treating when it is in the correct position.