Reduce, Reuse, Recycle.
Here's a nice easy one for you that reduces Junk Mail:
The Mailing Preference Service (MPS) is a free service set up 20 years ago and funded by the junk mail industry to enable consumers to have their names and home addresses in the UK removed from lists used by the industry. It is actively supported by the Royal Mail and all directly involved trade associations and fully supported by The Information Commissioners Office.
Registering your address and opting out of junk mail with the MPS is easy, it's free and it works.
BBC iPlayer - Panorama: Why hate junk mail?
Further reading:
The Information Commissioners Office: Stop Junk Mail
Guardian - Comment is Free: Junk mail – the facts
Roy Mayall: Going Postal