First things first. Posting should feel natural; not forced or strenuous.
Do you know your diagonals? Always "rise and fall with the leg on the wall." Your seat should be out of the saddle when the outside leg is forward/off the ground. The reason for this is, especially noticeable on a circle, because when you are on the correct diagonal, the horse's motion will "throw" you towards the inside of the circle instead of outside to help balance you during the movement.
During the act of posting, try not to over due it. It all just looks wrong when it is overdone. Think about wearing a pair of boxer shorts. When rising out of the saddle, only the part of the leg where the boxers hit (mid thigh) and waist should move up and forward. None of that shoving the legs and feet out in front of you to "get you out of the saddle". Hold your position with your calves and thighs and allow your horse's movement to push your seat out of the saddle. It helps to have your horse moving along at a nice pace--not a slow jog--to make posting easier.
Posting is sometimes used as a way to lengthen a horse's stride because posting allows you to be off your horse's back at times and not restrict is back. To do this when you are in the "down" part of posting, squeeze your horse forward with your legs and into the bit. This is also done during the warm up to help the horse stretch out and down and loosen up his back.
Opportunity is missed by most people because it is dressed in overalls and looks like work. ~Thomas Edison