Serpentines are great. Lots of circles (sometimes you need to exagerate the bend, but make sure its through their whole body and not just their necks)
A horse generally only gets hard if he's bracing against the rider's hand, so make sure that YOU stay soft :)
I would reccomend making your horse move forward while bending. You can do both bending into the direction you are going or bending your horses neck in the opposite direction you are traveling. This is a lot harder for the horse than bending into the circle. Counter bending may be a little bit of a fight at first because of this but it is very effective if you are persistant.