Again on the theme of the Guardian Media Group's mixed attitude to freedom, Fearghas points out that they are still employing Peter Preston, the editor that turned over a Foreign Office whistleblower to the police. Sarah Tisdall went to jail in the 1980s after revealing to the Guardian that Cruise missiles would shortly be arriving in the UK. Two decades later, Preston published a mea culpa.
That episode provides several lessons for whistleblowers. Tisdall was clever in how she supplied the documents to the newspaper, simply handing them in anonymously to be put in the news editor's in-tray. This avoided all the difficulties of later covering up contact (particularly via telephone or e-mail) between the source and newspaper; this is extremely difficult even for someone who understands in depth how these communications media work and what records will later be available to whom (short answer: assume all records of your communications will be available to junior security officials within government). Where Tisdall went wrong was in using a Foreign Office photocopier to duplicate the documents; even in the 80s, security copiers were adding invisible watermarks to copied documents that allowed them later to be traced. Because the Guardian had given copies to several journalists that could not be contacted when a Special Branch raid seemed imminent, there was no practical way to stop this information coming into the hands of the police.
It is difficult to know how far these watermarking technologies have spread. My best suggestion for a government whistleblower today would be to attempt to make contact with a journalist using a payphone, and arrange a physical meeting where the journalist can make notes based on a document without taking an actual duplicate. This also limits risk to the journalist and their newspaper, who can no longer be forced to provide copies of original documents.
Spyblog has just published a longer list of such practical considerations. I also wrote a chapter for an OSCE book on freedom of the media that goes into more detail on some of these topics.