Here is a good guide from Cryptome http://cryptome.org/cryptome-anon.htm

Tell the reporter to mail you at Mailinator or Lockbin.com and retrieve their messages by connecting with TOR.

Or tell the reporter to take out a classified ad in a free, public paper distributed in the nearest large metropolitan area.

The communications from the reporter don't need to be private or targeted. Everyone will know he's involved when he publishes, and he can reasonably encode questions by referring to the documents already sent. "MR X, CAN YOU PROVIDE FURTHER INFORMATION ON THE EVENTS DISCUSSED ON PAGE 13."

The communications from the journalist certainly do need to be private. Advertising that you are working on a story that will reveal big government secrets is a good way to be put under surveillance to find out who is doing the leaking to you. The journalist would not want to announce to the world that the process of receiving classified information is ongoing.

The ad can be written so as to look innocuous to anyone other than the journalist and the leaker, if those two can agree on a format for doing so.