Yes Sharon is definitely right there and so is cherry chops with the suggestion of sitting trot without stirrups. I would get someone to lunge you and hold a neck strap to get that relaxed sitting trot.

For the canter do not keep your knees pushed onto the saddle, nor your thigh. That is bound to make your leg come slightly upwards and therefore your seat too and that will make your stirrup come loose!

If you can just think of letting your seat relax into the saddle and spread over it sort of thing. then think of letting your leg relax and drop long long long and make it heavy in the stirrup, that will help a great deal.

It does depend on the level you are at right now of course and what sort of horse you are riding. Is it your own I wonder or is it a riding school horse perhaps?

I would personally think first of sitting tall but allowing your seat to spread over the saddle. Then concentrate on letting your legs grow longer and heavier into the stirrup.

This will make your seat a lot stronger and you might not even have to THINK about what to actually DO with it.

Then, providing you HAVE managed to do that (tall erect body, spread seat and long heavy legs) you just take your outside leg back a bit and nudge firmly. No more than that, and it should work.

(providing your hands and arms are not stiff and preventing the horse moving forwards !)

Easy for me to say !! haha