At the local recycling centre... "Please sweep up. Thank you'.
So please and thank you, so as many polite words as instruction words.
What is nice about this is that the implication is that you sweep up your own mess. But if course this could be read as a direct instruction to sweep up mess, irrespective of whether it's your own or not. Even if you don't have any rubbish...coming across this sign seems to ask you to muck in.
