I would suggest long legs and weight in your heels. It may also be that you're concentrating on loosing your stirrups too much which might be causing you to loose them. Adjust the stirrups to your correct length for flat work (approximately they should come to your ankles) and put your feet in the stirrups and make sure you always have proper weight in your heels.
I find that practicing this outside the saddle works as well. Try standing on the ground with your feet shoulder length apart with some bend in your knees and push your weight in your heels. Now try to post by keeping a straight back and straightening your body up and down. This is what my coach taught me hope it helps! :)