My instructor told me to ride on my small toes (roll feet outwards) and stretch your heels to your horse's hocks with your toes pointing to your horse's nose
Really focus on keeping your heels down! Eventually it will become a habit.
Also, remember the "rule" which says that there should be a line going from your shoulders, through your hips and down to your heels.
You should also ride bareback or without stirrups, to improve your position and balance :)
Hi Rebecca, I was always taught to keep a very deep seat by keeping heels very low. We would do a lot of no stirrup riding to encourage the deep seat and keep legs still. Hope this helps