In last week's Lost, just as Juliet and Sawyer were boarding the submarine back to the real world in 1977, Sawyer told her to "buy Microsoft."
Imagine all the things you could change if you suddenly found yourself thrust back in time: You could destroy the planets of your enemies, like Star Trek's Nero, or kill the mother of your future nemesis (or at least try), like the T-800.
Or you could follow this awesome new T-shirt, available at Topatoco, with simple instructions on everything that you could do to affect the future.
As a bonus, the T-shirt can serve as a guide to rebuilding civilization if, like the Galactica crew, you find yourself stranded on a pre-technological planet with no aluminum or cell phones.
Keep it with you: After all, you never know when you're going to be sucked into a black hole. (Thanks to Cinematical for the tip.)