It took me a while to get it, but to figure it out, what helped me was this:
My trainer told me to sit and lean back. It doesn't matter if you're not strait at first when you are learning, once you "get it" you work on looking nice and everything.
So basically lean backward and try to roll your hips with the movement of the horse. (Easier said then done, I know, but with time you'll figure it out) Leaning back helps, because you pretty much get the feel of the horse's movement much more easily.
Also make sure you legs are at the girth and not too far back because it really helps when they are stable and in the right position.
I hope this helped! If you have more questions just ask! :)
A ride a day keeps the worries away!