I'm going to point you to this thread: HorseHelp: Building up your horses back, In-hand and Under Saddle
And then I'm going to give you a little bit of information that I know.
Ground poles, walking over them, trotting over them. Working up and down hills, long and low work, any kind of lateral work like leg yielding is great! Backing up is good for the hind end. Be careful about backing over poles.. it's better to teach them on the ground first and then do it under saddle.
Transitions from walk to trot, and then using canter to let the horse stretch out.
But good correct and consistent riding will get that muscle coming.