Long and low or long, but not low! When riding in one of these positions the back muscles relaxes (they automatically tend when pressure is applied on the horse's back because the back muscles are made to protect the spine). When the back muscles relaxes they allow the vertebraes to rotate naturally. Just remember the importance of not bending in the 2nd and 3rd neck vertebrae, this will couse unnecessary load on the front leg because the horse can't lift it's withers.
Lots of pole and hill work making sure he working actively from behind
Using the Pessoa Training System! Lowering your horses back will improve their muscles in that area