Pilates and stretching!
Core strength is really important, especially if you've got a thin, short-wasted build like me. Your lumbar spine takes a lot concussion from things like sitting the trot, landing from a jump, or working around the barn, so having strong and supporting core can save you from back problems in the future.
Flexibility is something I can't stress enough. You can build all the bulk you want, but if your muscles are tight then you can't use them! (I laugh at those body-builder guys who can't rest their arms at their sides!)