Hill-work and working them long and low. The only guaranteed way of building topline is consistent, correct work, with the horse bringing his hind end underneath himself, therefore lifting his back and allowing him to reach forwards and down into the contact. Then with hill-work, literally working at a walk up and down slight slopes will make a significant change in a short amount of time. (Making sure that you're asking for a decent walk, not just a plonk!)
Lunging in a Pessoa and click the ends on the wither not between the legs.
working over poles and cavaletti will also be helpful