Are you riding hunt seat? Are you in two point? Do you lose your stirrup when cantering?
Here's my advice, and it's more from a dressage point of view.;
While trotting or walking practice dropping and picking back up your stirrup.
While trotting and posting, practice repositioning your stirrup on the ball of your foot, and when you like the feel of it, drop down "into" it so that you feel your weight going downward and back toward the rear feet of your mount. IN fact, whenever possible, try to mentally join your heels to his rear feet, I mean kind of point your energy down and back. This helps keep you feet under you in the all important lineup. Please do not put your feet forward into a brace or a chair seat. It is a habit that is hard to erase. I can see it if you want to do it for feel in comparison to then trying to stack up your body over your hips/heels in the correct way.
Another thing is to kind of "pet" your stirrup. I mean feel it with your feet and put some pressure onto your ball of the foot onto your friend, the stirrup.