Not lunging on them. You ride the hills. Up and down. You back them up the slopes (again NOT too steep). It really helps to build the muscles in their butts.
Lots of trail riding with hilly ground will help buil muscles.
What do you currently do with your horse?
A hot walker is something you hook your horse up too and it walks them in circles.
If you Google, bending exercises for horses you will come up with a nice variety of things you can do.