I've always had the "Please don't knock, baby is sleeping" note on my door from when DS was born. It keeps everyone away LOL.... but I love it. You could just run with that. Friends will not be offended & I'd be suprised if they still nocked. As for Aust post etc they usually just drop off at door or not in box.

Though I do have a "please no junk mail" on my letter box....starting to wonder if those filling it up with crap can read!!!!!