Take a small pile of books, lay a mat (car mats usually work, or piece of wood) across the top on a hard surface stack the dog with the front paws on the books, so head higher than bottom facing 'uphill' and reinforce the stand command.
I don't know how or why it works and it always seemed to me that the bottom should be raised not the front but it does work this way around! It stops the dogs going backwards, or sitting, and you are not straining any part of the dogs anatomy to do it. works wonders for 'sitters' too.
