Ride a few minutes w/o stirrups, then pick up your stirrups by lifting up your toes. While riding, think of lifting your toes UP. Wiggle them, lift them up, to the point where you almost drop your stirrup. Play at kind of "peddling" on your stirrups as you would a bicycle, with each stride of the horse. Periodically do the above to check in with your stirrups and be sure you haven't zoned out inot board stiff ankles. At times, bounce off the balls of your feet during posting, exxagerate riding off your feet, then just relax and think of your stirrup carrying no more than the weight of that leg.