Last week I had the opportunity to ride with an unbelievable dressage trainer and I can't believe (ok I can...) how much it has helped my jumping and flatwork alike. We spent a ton of time on position and rhythm and transitions. My horse is going amazing this week, soooo light to my aids and much more willing, which I am sure is due to my being a lot more balanced and clear with what I am asking for.
I know people on this forum talk about dressage as a foundation for all disciplines a lot, and it is soooo true. All you hunters and jumpers out there, if you have the chance to take some dressage lessons, DO IT!