Pole trotting is a great way to activate the behind. And I always tell everybody to work on tempo changes. Example: if you ride in a 20x60 arena, make a tempo change every 5 to 7 meters (not only trot-canter, but also trot-medium trot) Or every 6 steps the horse makes. Build this way of training up. Don't just do it for 40 minutes straight, your horse will not gain muscle in the correct way!
The first thing for me is to work on flexibility, the more supple your horse is the easier it will be for him to build muscles so basic stretching exercises are always great.
I also love to rein back on small inclines, only a few steps each time, but it works wonder. You can work on elevated ground poles, by increasing the difficulty when your horse is confortable with the exercise. Gymnastic with small jumps can also be great!