I'm definitely not an expert by any means, as I'm still learning this myself, but I'll relay what my riding instructor told me, just to see if it helps.
She always told me to use my hamstring muscles. Feel like you're a weeble-wobble toy, and that all of your weight is in your butt. You're completely balanced in the saddle then. Then, when you start posting, try to concentrate more on your hamstring muscles than your feet or ankles. To me, it feels as if your legs are just a simple-action gear. When you rise, the angle between your thigh and calf opens up. When you fall, the angle closes. Sometimes, when my legs slip forward, she tells me to imagine my heels extending over my horse's butt. Sounds pretty dramatic, right? But I swear that that always gets them back to where they need to be! And usually you're only an inch or so off!
One problem that I continually have is that I put all of my weight onto my big toe. Try to relax your feet and ankles, and distribute that way all the way through your pinky toes. Just remember that you're just using your feet passively.
I always hum a song while doing this, as it helps ("In the Hall of the Mountain King" is a pretty good song to post to, as long as you don't speed up the tempo too much!). Sometimes, when you completely ignore your rapid mind going, "Too much this! Too much that!" You'll find that you get it down really quick.
I'm guilty of a busy mind, so no worries!
I rode just the other night, and my leg slipped WAYYYY out during the rising trot. It was because I braced my leg, and didn't bend one of my knees (which I was guilty of doing a LOT before taking english lessons). It just went whoop! Right out in front of me. I had to laugh because it felt like my leg was a croquet bat, and I was swinging it! Completely and utterly wrong, I know! I definitely lost my balance on that one, lol!