This has been a subject that I have been thinking about for a while and I recently read a post from another blogger who has been struggling with a similar 'relaxation' issue.
I think the single most useful tip I have learned recently that has improved my canter is the phrase "let the horse canter on and you get left behind". It helps to repeat it in my head "you go on, I stay back" in rhythm with the canter stride. What I started to notice was that while I tend to lean forward in a more 'hunt' seat, the leaning wasn't the root of the problem. The root of them problem was in my pinching hip. It was like trying to pry open a rusty hinge to get me to lean back. As I would lean back, I would feel the tension in my hip/groin area. Finally, I relaxed my hip angle open and all of a sudden I was more connected to the stride and felt more independent and secure. I felt like my upper body could stay stationary while my hips rocked with the motion of the stride.
The more I have thought about it, the more sense it makes. In hunt seat equitation, you want your hip angle closed and you keep your stirrups shorter (we are comparing to dressage here, by the way). The hip angle is smaller. Its more of an acute triangle. In dressage, you want that hip angle open more like an obtuse angle. The more open the angle, the further back the body leans (or more upright, however you want to think about it), the more 'seat area' you have available to work with. The goal of dressage is to use the seat more effectively, thus a more open hip angle provides a more effective seat.
This more effective seat is also known as an independent seat. Having an independent seat means that you can further flex and relax the rest of your body at will. So, if you have a tendency to have floppy legs or heels that refuse to stay down, think about your hip angle. These problems are impossible to correct without an open, relaxed hip angle.