Flat work, lunging with side reins, cavaleties
Cavaleties up and down transitions hill work and framing and getting the horse in the bridle
Long and low neck stretches at the trot up and down hills
Trot poles with a longer reign , really look for that stretch! Pole exercises that cause your to use my impulsion also help, you could raise one side of each pole for that extra push.
I have always tried to at least once or twice a week to do low and balanced work with my horses and their toplines are looking pretty good since I started that routine.