Mark, here is one way that I have seen it done.

Dig a small hole, at least a foot deep, and fill it with water.

After the ground softens, stand the rod in the hole, and push it down 
as far as you can. Initially, that will not be very far.

Now, pull the rod out, add more water to the spot where you pushed it 
down, and push the rod down again.

Keep doing this until you have gone as far down as you can. Then, still 
keeping things as wet as possible, bring out the sledge hammer.

Depending on your soil conditions, I have seen people push an 8 foot 
rod at least 5 feet into the ground that way.

-- 
Mike Duke, K5XU