I'd write in big letters:
Dear martians/Xen lifeforms/blisk/other:
Hello. We've discovered nuclear weapons, and blasted ourselves into little red puddles. We made Stonehenge, the Pyramids, and we once had some nice gardens in Iraq. Then we made religious fundamentalism, political feuding and daytime TV, and kinda screwed things up. Oh and while I think of it, I hope you're wearing your bio-suit, we've earned our resistance to the bacteria here. There's probably a few Humans still kicking about: I would try shouting "jibba jabba" over and over while acting effeminate, or try looking in your Improbability drive.
Thanks for visiting, and please don't let the several tonnes of supersonic space debris hit you on your way out.