Glad you asked...
That's a weird question and depending on the circumstances, it could border on illegal. Dogs mount for different reasons- sometimes to show their dominance and sometimes to mate. If you have a dog that falls into one of those two categories, getting on your hands and knees would be a start.
Then again, this answer might help you...
Start out with a command word for your dog. Use the command word and show the dog what is expected of him. When the dog successfully mounts you tell him he is a good dog and reword him.

