The Pokemon has to hold an item (Blazikenite for Blaziken, Lucarionite for Lucario), and the mega evolution can only happen in battle. Once the battle is over, the Pokemon devolves back to its normal form. --- The official rad dude of the Pokemon X board.
A currently unconfirmed number of Pokemon have the ability to Mega evolve during battle if they are holding a certain item and they must also possibly meet certain as of yet un-revealed criteria. Only 1 Mega Pokemon per team at a time. --- Gamertag: T0tal Carn4ge Star Craft II Bnet name: ChaosDragon # 298