Run it as slow as you can, RPMs are not needed. Sometimes it helps to "peck" at it, and raise the auger now and then to clear it. If you hear it hit a big one, give up and get the hand tools out - it won't bore through rocks. The holes I dug all had a layer of smallish, sharp-edged stones laying between the topsoil and the clay. These would get in the auger and lock it up. When this happens, you just have to pull the auger and get the digging bar out. You will want a good hand post hole digger to clean out the holes.
Basically, it's a game of trying to keep the force on the shear pin from getting too high, and shock will pop it faster than anything. Buy a big bag of shear bolts and nuts (unless you've got a slip clutch on yours).
And remember, if your putting in fence posts, once you trim off the top no one will be able to tell if it isn't in full! /forums/images/graemlins/blush.gif Just don't put two shallow ones in in a row.