I know it's been said alot, but hills are great for getting their top line back..
Ride uphill in sand or deep ground, down hill would work too
I use hill work, first week of just walk asking them to push from behind and be in a low stretch, then next week alternate from trot to walk on the then third week only trot, still pushing and stretching.
The best thing for the back is hillwork, hacking and correct schooling, as Adeline said. Just time and correct effort. If a horse is working well and not building muscle slowly, I would be getting it checked over as it would signify a problem to me.
Just correct basic dressage, aiming to get the horse really working through it's back. Long and low with a connection, transitions, and basic lateral work, turn on the forehand and leg yielding, making sure the horse is using it's back correctly during the lateral work.