School a lot of dressage movements, even if you have no intention of ever riding a dressage test.
Try doing some leg yields from a 10m circle to a 20m circle; changes of bend, figure 8s, or serpentines; trotting on a loose rein (either in a circle or around the perimeter of the arena), all the while trying to get your horse to stretch down and lift his back -- it should feel like riding a suspension bridge.
Also try do do a lot of walk work -- the walk is generally neglected and helps the other gaits immensely while building muscle and not unduly taxing the joints. You'd be surprised how much 15-20 minutes active walking per ride can help!
Try some trot lenghtenings across the diagonal, as mediums and lenghtenings help with the collection needed to rock back onto the hindquarters and jump a course in a balanced, rhythmic, uphill canter.
If your horse is lethargic, do lots of transitions, particularly walk-trot-halt and variations of. If he's hot, try to use your seat to slow his tempo. Post big and try to get long, slow strides, and remember that you control the tempo, not him (or her).
But the most important thing is to ride your horse forward all the time. The walk should march, the trot should swing, and the canter should spring!
Remember that jumping is just flatwork with a bunch of sticks in the way, so even if you're only jumping 1/5 of your total ride time, make your flatwork productive to improve your work over fences too!
"Horses are self-portraits of their riders. Autograph your work with pride!"