Like so many learned skills, it's about repeating in a consistent fashion until you don't have to think/worry/ponder about it. This shouldn't be news to anyone. One factor that may not be self evident is the tightness of the retention mech. Many clipless newbies who worry about falling while trapped to the bike set their retention very loosely. Ideally, the release motion will be quick and positive. A loose pedal may allow slop and opportunity for the heel to move off the horizontal. What should have been a quick, short flick of the heel becomes a spastic thrash with a bad result. I don't think this is OP's problem since he's been using clipless for years. May help some others.
