Its tuff to give you any pointers without actually seeing you do the pass. But off the top of my head i'll just say that its mostly about the timing. It'll take a while to get the timing down. Its not really something you can just learn and do. It takes trial and error. As far as the action of the retaining hand is concerned (the right hand in my case), like mark said you want to let it fall onto fingertip rest. There are other versions of the pass that leave the coin in other places and im sure some of them are great but for me it looks best and the hand looks most casual and relaxed ( like its supposed to) when you go straight into fingertip rest and let your hand hang limp like liberace.
Here is a video of me doing it several times just incase you'd like to see what i'm talking about.
http://www.youtube.com/watch?v=EvuGKxgKFTI
In the video i almost instantly reproduce it so there isn't much time where i'm holding out and have to make my hand look natural. But Hopefully you can get a sence of the timing. I let it burn for a somewhere between a quarter and half a second right up in the crotch of my thumb then close my hand pull away and let it pop onto fingertip rest. You might also notice i sort of rock the coin forward then back as a put it into my hand. I do that to make sure that it catches the light and really burns the image into your brain so that i can get the best rov possible.
Hopefully you can get a video up and then we can all see what needs work. Good luck.