Funny this thread would be made and I see it first right after I bought a 6' galvanized steel ground rod and plan to pound it down.
One method I read about is to push the rod down as far as you can, pull it back up and add water to the hole, repeat, up until the point where you can no longer press down by hand, then do the same with a hammer.
Supposedly you can work the entire rod in easily that way and I sure hope so because I don't even have a sledge hammer right now, just your average hammer.
Worst case I end up hitting rock, or can only get it so far down and have to wait on borrowing a sledge hammer later on.