Other than everything mentioned lateral excercises are great. Especially shoulder ins and haunches in.
I have a horse with this problem! Lots of circles at walk and trot and some canter spirals (gradually making circle smaller and then making it bigger again) leg yielding, trot poles to make your horse really have to open up, turn on the haunch and forehand, lengthened walk trot and canter, basically a ton of bending stuff don't just stay on the rail! I do this every ride because my horse is stiff but he always opens up after this!
Think bending , flexing and moving the feet while moving around you or your leg.
I loosen up the hindend by moving around in small circles and asking for them to move the hindend over both directions. I will also ask the same of the front. If my horse isn't stepping over with a leg then I look for why. Crossing the hooves as the step sideways with either the front or back. They must learn to stretch while relaxing.