It is very difficult to strike that "perfect" balance...I think that posting with your reins loose is a good practice for you since it forces you to balance with your seat and not with the reins. But once you've got that down, try gathering the reins up in stages -- maybe start from completely loose, then gather a little more, and a little more, until you have full contact.
As far as I've ever been taught by my trainer, you shouldn't be pulling on the horse's mouth, just maintaining very slight pressure there so that the reins aren't going back and forth between loose and tight. But it just takes practice that's for sure! Does your horse put his/her head out really far and low or high and in like a dressage frame? Sometimes the "low and far" heads are the hardest to handle with the reins, my mare does that because she hasn't been fine tuned in dressage quite yet, and she always seems to be pulling on the reins because of it.
Maybe you could try having someone longe you and your horse while you ride without reins and practice balance, then gradually gather them up a little at a time? Then at least you're not worrying about steering and all that, you can just focus on the balance and the reins.
"The times when you have seen only one set of footprints in the sand, is when I carried you..."