I used to have the same problem and my instructor told me it comes from the hip and not the ankle or knee (at least in my case). So I had to work on rotating my hip at the pelvis and obviously work on a poor seat (it's all connected!). Evenrually it all came together but I still can't ride like my instructor
Think less about your ankles and think more about your knees, allow your knees to relax and come away from the saddle. Also think less about pushing heels down and more about lifting your toes.
I've found it is easier for people to keep their toes in if they think heel out, that way they straighten the foot. It seems to be an easier action with the same effect.
ok thanks everyone but now confused is it my hip my knees or should i just push my heel out ,I do have long legs as Im 5'10'' and I ride a skinny 15hh arab which does not help im sure.
you could try wedge stirrup treads with the thickest end on the outside ,they will keep your thigh in the correct place and so your toes wont stick out!
If your toes are sticking out perhaps your legs are not as relaxed as they should be. Try some work without stirrups and really stretch your legs down and practice riding by balance alone, and not gripping up. When you take your stirrups back keep your legs just as relaxed, and, as someone else has said, push your heel out. Good luck!
without seeing pics of you riding it is hard to say what causes it - could be the way you have your thigh on the saddle. Try grabbing the back of your thigh off the saddle, it will move the contact of the saddle on your thighto a better position that will in turn rotate your whole leg and turn your feet in.
Stand up in your stirrups, and turn your toes and knees in like if you were knock kneed. Then try and sit back down without moving your legs. That should get them in a good starting position then you just have to try and keep them there!
You want the front of your thigh snug against the saddle rather than the back of your thigh.