This one is extreme, even for me, and quite near my limit, but worth a shot.
Go to public toilets and look for a stall with garbage can that got used toilet paper inside it, with visible poop on it, that some unknown stranger used before.
Take some of that paper, you don't have to touch the poop, and wipe your own butt with it making sure the stranger's poop does touch your butt hole.
You don't have to poop before, the dare is just the act of "sharing" paper.
Here is my own proof of doing that:
http://www.youtube.com/watch?v=iTF-3GBIa5k