What everyone else said plus ride in A LOT of circles.
Go to any top training barn (with the exception of saddleseat or gaited horses I guess) and the pro's will be riding in circles over and over and over; day in and out, day in and out. Walk, trot canter.
Becuase circles make a horse soft laterally and vertically in an easy to understand, more forward method. They have to flex, soften, and go forward in a balanced gait.
Boring, but true. I spent many a summers riding a freind's crazy jumper in circles and never jumping a darn thing! When she finally hopped on and jumped him he did excellent, he was soft and flexing well, he was relaxed and comfortable... of course a month or two later he was back to crazy becuase all she did was jump him again, but thats besides the point
Point is, do lots of circles. While more boring than racing barrels, jumping jumps, or chasing cows its the best thing yo can do for a horse... then take a nice trail ride, or a relaxing walk around the yard if you don't have a trail to spice things up when your done!