I'm surprised no one mentioned the Pokerus virus.
Anyways, Pokerus is a nifty little affliction that will make your mons gain twice the EV values whenever they battle if they're infected. this can cut the time necessary to EV train literally in half... AND the benefits don't go away when the pokemon loses the affliction, the x2 EV effect is permanent.
but you should know that Pokerus is extreamly rare however... theres only a 1 in 21,845 chance of finding a mon infected with it in the wild/ being bred and having it.
here's more on the subject: http://bulbapedia.bulbagarden.net/wiki/Pok%C3%A9Rus