What I do is I feel for the outside front leg's movement. Try to picture in your mind when the horses leg goes forward, and just rise with it. I'll sit for a couple of beats while I figure out when that leg is moving forward, and once I find the rhythm I post. It might help to have someone lunge you at the trot and close your eyes to really pay attention to the rhythm.
I'm trying my best to explain how I get it, but I'm pretty sure that isn't all that much help haha. All I know is that is what I do and I get it 100% of the time without having to look.
Good luck :] & I'll try to think of better ways to explain it.