Assuming there is no law against defacing your money (there probably is for fly posting) just write "bitcoin.org" on all the debt notes that pass through your hands then everyone who uses the money will see it.
Thanks a bunch for the ingenious guerrilla marketing idea. I've just gone ahead and done this for all the euro notes currently in my wallet. Here's photographic evidence as a sample:
I experimented a bit with different writing instruments, and in the end would recommend using a pencil. It works and looks better (at least on euro notes) than a ballpoint pen, doesn't permeate to the other side of the note as a felt tip pen would, and is erasable in case the payee gives you any grief about your illicit graffiti
Yeah, the erasability is important. While it's not illegal to write on Euro notes, notes that have been written on are no longer legal tender, so it's good idea to be able to erase that should someone not want to accept your notes.