Walk trot transitions are perfect, try to be as smooth as possible and don't lose the forward energy either going up or down.
Lateral work has done wonders for my horse, exercises such as leg yield, haunches in, shoulder in, etc. But always have forward movement, think impulsion, not speed. Also try lots of transitions, working from walk to halt, walk-trot-halt, etc.
Try doing like half raised poles. Where you only raise one end. But have say 4 of these in a row. In my experience it has helped to collect my horse and make him use his hind end a lot more.
Flexing its neck and a deep seat
setting ground poles up to make 4 90 degree corners and then riding them at a walk, trot, and canter will really help your horse to get on his hind end. It's a little hard to explain, but i can send you a picture of it if you'd like!