(Shakes head) Denno, Denno, Denno..you should know this.
Battle wild Pokemon to gain EVs, and/or use Vitamins/Wings. Use Berrys that "lower the __ stat, but increases happiness" to lower the EVs.
Pokemon give off EVs give off different amounts with the species, but Vitamins and Wings will always increase the stat by 1 EV. EV reducing Berrys will reduce it by 1.