We had a thread about this recently
http://www.accringtonweb.com/forum/f66/where-34116.html
You can buy door signs on ebay dirt cheap, but since my front door is in an enclosed communal stairwell, I have a paper one I produced on my PC. It is only aimed at religion pushers, so I still get those fuel supply chaps - just say I check with Uswitch - go away.