Gameplay » Game Actions
If an action is *optional* (e.g., playing a Trainer, attaching an Energy, attacking your opponent's Pokémon, etc.), and you forget to take that action, you may ask your opponent for permission to take that action. For example, if you attach a Grass Energy to your Pokémon, and you wanted to attach a Fire Energy, you may ask your opponent's permission to take back the Grass Energy and attach the Fire energy instead. If he or she says no, you're stuck with the Grass Energy.
Source: Organized Play Announcement (2007-03-14)
Gameplay » Game Actions
If an action is mandatory (e.g., draw a card at the beginning of your turn, take a prize for knocking out a Pokémon, place damage counters for an attack, etc.), and you forget to take that action, you do not need your opponent's permission to do it. However, you should immediately call a judge so that things don't get muddled.
Source: Organized Play Announcement (2007-03-14)