First up, Phd by law here(Australia) have to have a slip clutch, and to be honest after using one like that for years I couldn't think of anything worse than having sheer pins! With a slip clutch, you can keep the rpm up, and when the auger gets caught, the clutch will slip a couple of time's then pull up the rock, the worst is roots, if you hit a big root, the auger will get stuck, but often what you can then do is just lift it and use the 3pl to break the root out. ( if your tractor is big enough)
As for augers as someone else has said, go as small as you can, the bigger the auger the more engine HP you need to move the dirt, the length also plays a factor, a long wide auger will take more HP than a short wide auger.
I have had success drilling a small hole then enlarging with a bigger auger, but it can be a bit hit and miss, if you hit a rock, or root the larger auger will skid off course
The best thing we have done lately is buy a hydraulic drive head, we mount this on our fel , this allows us to put pressure down on the auger when we are drilling, when you hit rock it just drills right through it, it's slow but haven't dug out a single hole since we got it. I'll try and find some photos .