I find this is best for the lower back (there's some bike racks near my house that are perfect for these; sometimes you can find ladders or narrow stair-wells to do these):
I do these for more mid-to-lower back, but with a sign-post or a door frame/handle (usually I anchor my feet to the base of the post):
If the other 2 methods don't work, I find a park-bench that's bolted down, and do these on it:
