Lots and lots of long trotting and going over ground poles...and make sure you're feeding right for them to build the muscle back! It's a long process, but very rewarding!!!
Ground work- Backing up in circles, straight, slow backing with a level head set. Mounted- walk/trot/walk transitions and a long-strided, ground covering walk to keep the back muscles supple. This over the course of a couple months really helped my horse's top line.
I am in that boat right now! I got a chambon to help encourage him to stretch his neck down and work his back. Also I focus on riding him on the bit, really stretching through his whole back/neck and getting him reaching beneath himself with his hind legs. As was stated, feeding well, going over poles, and transitions help.
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