Work long and low. Here is a short video that is pretty good at introducing this concept to at least get an idea of what it is:
Some people confuse long and low with just letting a horse's neck stretch down. True long and low depends on impulsion and engaged hindquarters, not just on a low neck. And when long and low is useful depends on the horse. A lot of people like to try for long and low right away. My horse doesn't work that way. When we first start riding I don't care what his back or neck are doing, I just want him to go and find his rhythm and get moving. I want an even trot, I want forwardness. We trot and canter on a longer but not long rein until he has a few good snorts and gets a brain and a motor and THEN we do long and low in circles and serpentines all over the place, both at a trot and a canter. This works really well for us. A lot of people find that serpentines with trotting poles is a great exercise to get a horse long and low, stretching, using its back, and engaging the hindquarters.
Work according to the dressage pyramid, eventually up to collection.
Feed hay on the ground. Hand graze them while walking. Ride up and down hills.