Lots and lots and lots of trotting transitions. But it needs to make sense.
I don't find putting them on a circle forever helps, but trotting large and then doing some figures at the trot would help your horse gradually gain balance. As would putting down poles and having him walk over them, then trot over them. Uphill trotting is good too.
I trotted my horse for 7 months (walk and trot, but no canter cause he hasn't learned it yet) and now he has a beautiful uphill balanced trot.
Just miles in the saddle.. but good miles.
"Strength is the ability to use a muscle without tension"