I also use the hinged prop rods that attach to the radiator support rods (you can see pics of them in my 'more pix on Photobucket' photos).
The trick I used to keep them from falling/slipping is, I placed a strip about 1" wide by 2" long of black 3M skateboard grip tape under the hood on each side. The rubber tip on the end of the rod is placed against the grip tape when either side or both is opened. It has NEVER slipped or fallen since I added the tape.
