When I was young, I used to use a piece of PVC pipe. Put the rock into the end near your hand, and use an over-the-shoulder swing. The rock will fly out VERY fast, and with practice can be very accurate. There is an ancient weapon used by some tribes in, I think, South America much like this called an axlotle (I KNOW I mangled that word
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EDIT: I found the correct word is "Atlatl" and can be found below.
http://www.atlatl.com/mechanics.html