Posting shouldn't hurt your knees. If it does, then you are pulling yourself up by your quads and the muscles around your knee .Thats' kind of like "hinging" yourself up off your knee, with your lower leg hanging straight down from the knee and the weight of your body not going down into the lower leg. This is NOT correct.
( If you ski, you will have a similar feeling when you lean too far back in your boots and you have to tightenn the muscles on the front of your legs to keep yourself upright and you will always be "behind the motion". It makes your legs really tired and hurts your knees. But if you stand correctly balanced over your skiis, your quads don't have to work half as hard.)
You want to be positioned such that you can have your weight go down thru your knee into your lower leg and ultimately to the stirrup. You post off of the whole leg and instead of heaving your body up and back off of your knee, you will be more as if you are standing on your own two feet and just gently lifting you pelvis up and forward. Your knee will flex a little as will your ankle, all parts doing a little bit of "accordioning" to lift you and put you gently down.
I like what another poster said about letting the horse lift you. And that's good, but just don't heave yourself off the knees. Stay upright and such that if you horse magically disappeared from under you, you would land in a standing positions (and say WTF?!!)