I had this problem as well when I began riding English. I haven't had many lessons from an instructor in an English saddle, so I just took what I could from my trainer, and a little from my own 'figuring it out'.
Getting your legs underneath you is the first thing. I don't know about you, but I had an awful chair seat when I first started English. You can take your feet out of the stirrups and let them fall naturally, then bring your heels up without moving the position of your leg much to see about where they should be.
To keep them solid (and straight below you), my trainer told me to sink my weight into my heels. When I put more weight in the stirrups, it helps keep them still.
When you post or go faster, I found it helpful to almost imagining pivoting from the hips and locking your legs into one position. You shouldn't be posting off your legs - you want to keep them still, and move off your upper body. For me, it really was about developing the muscle to be able to transition between western and English. At first when I was focused on staying still and locking my legs and all that, I was really stiff and tense, but when the muscle memory developed, it's much easier to be supple and relaxed but solid at the same time.
If my explanations made any sense whatsoever, I hope they helped. Again, I'm far from an expert, so I'm sure someone else can offer you a more straightforward answer. This is just what helped me.
And your English stirrups should generally be just about the length from fingers > armpit when you measure your arm against them.
Last edited by Paradise; 07-18-2012 at 12:44 PM.