Being half of a celebrity twosome has its perks, not least of which is starpower greater than the sum of its parts. But there are certain rules that should be observed. Do not, under any circumstances, appear in a treacly magazine spread rhapsodizing about your love, or you will be broken up within a month. And please, resist the temptation to get matching tattoos, especially each other's names, as history has taught us that even the most inspiring and rock-solid romantic unions—Lindsay & Sam! Chris and Rihanna! Britney & K-Fed!—cannot survive the curse of the matching ink.