If you've got one of those Kelty Noah-type tarps, the most straightforward way is to hang it in the rectangular configuration, and stake it down not at the corners, but about a quarter of the way in from each end. Then take the corners, fold them inward and stake them down below your hammock suspension. From the side, it will look hexagonal, since the "doors" will appear to angle upward.
You can do something similar with a hex tarp, but you have a lot less material to work with, so it's not usually as effective.
"Can't we all just live in trees and hammocks?"
-- Sam Gribley, My Side of the Mountain