There are many threads here about that. What works best for me is a swivel socket or wobble extension (you need a little bit of angle), and enough extension (~6") to get your ratchet handle right at the back end of the starter. A flex head ratchet helps too if you have one. Be sure to leave the bottom bolt in to take the pressure off the top one. If you have 4WD, take out the two bolts mounting the front diff assembly and let it droop down. It will move just enough to make access a LOT easier.
Some people have reported going through the wheel well or reaching over the front cross member with a very long extension, but those didn't work for me.
