FTA: "There’s another option I didn’t originally mention here — leaking over mail. Investigative journalist Julia Angwin of the Wall Street Journal points out that physical mail, dropped in a random post-box with a bogus return address, is perhaps the best way for anonymous one-way communication."
DO NOT DO THIS! Every printer leaves a microscopic fingerprint on every printout. The printouts can be traced back to your printer. If it's an office printer, that still narrows it down considerably.
Even electronic documents can have watermarks, etc. For photographs, there's the EXIF information, for instance. If you want to share a photo, pipe it through "djpeg | pnmscale 0.99 | cjpeg -quality 90" first. It will get rid of EXIF, and also re-compress the image, changing its signature.