When I first tried posting without stirrups, I could not get out of the saddle to save my life. What I did until I had enough strength on my own was to hold my reins long to I could put my hands on the pommel of the saddle and push myself up with each beat. I was cheating, but quickly got the idea of what the motion was supposed to feel like. Once I got stronger and was able to do it on my own, I realized that you need what I will call positive friction to post. I honestly don't know if it correct or not, but when I distribute my leg contact from mid thigh to my calf, from about 6" above the knee to about 6" below it. I can post no problem and leg stays on and dead quiet. Works great, and I feel very secure. It has since translated into a pretty secure jumping position too.
You just have to see your distance...you don't have to like it.