Place a diagonal pole or small jump/cavaletti on the middle of the ring and canter over it turning the opposite direction
Putting a pole in the middle of the school/field and starting with a trot. Start on one rein then go over the pole and turn in the opposite direction then you can do the same but in canter:) this will hopefully teach the horse to land on the right leg.
Do figure eights using trot and canter. Canter the circle but trot through the middle area of the figure, then transition back to canter to go the other direction. Practise until there is barely any trotting needed to change, but poles are great too :)