You might try working with a different sized coin. I find it easier to work with half dollars than quarters. Plus, half dollars are more visible.
One thing that might help is to make sure that the coin is not in the middle of the palm. You want it further towards the heal of your hand. That way you can grip it with the muscles at the base of the thumb and the base of the pinky.
As always, practice, practice, practice. Keep a coin palmed in each hand as much as you can during the day. Perform as many tasks as you can that way.
One other resource is Dan Watkin's site: http://www.coinvanish.com
He has a great tutorial on the classic palm.
I hope this helps.