The basic rule as already posted is to drive the rod straight down. If it won't go straight down, drive at 45 deg. If you can't do that, bury horizontal.

I once spent an hour driving a rod down and it seemed to be going fine, other than really slow. I got done and turned around, then tripped on the point of the rod sticking straight up behind me. The rod went down a foot and ended up doing a 180, after hitting rock.

Too many times ground rods are cut at 4', (or less), because someone didn't work at it hard enough. (I'm sure I'm not the only one that's pulled up a two foot ground rod.) Sometimes they just won't go, granite and sandstone are two examples I can think of where they won't. Sometimes the ground just isn't very conductive.

One option is to use a chemical ground. You can buy them, but I've probably built a half dozon of them over the years. Pretty straight forward. I get the cad-weld/wire at the electrical supply and the pipe and bentonite from a pipe supply. I've used rock salt to fill.
http://www.erico.com/products/ChemRod.asp

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