Ash says a Pokemon's name and then whips out his Pokedex to check its data.
Take two if it was a Pokemon he already encountered.
Take 4 if it was a Pokemon he already caught.
Ash gets another badge.
And May or Dawn gets another ribbon.
Ash comments on how Gary is always one step ahead of him. Take two if Gary appears.
Ash explains to the person that he just met that he wants to become a Pokemon Master.
Brock hits on a pretty girl, only to be dragged away by Misty/Max/Bonsly/Croagunk.
Ash uses Pikachu in battle, even when the type is a disadvantage, and the other Pokemon is stronger then Pikachu.
Two if Pikachu uses Thunderbolt
Take a drink if Ash wins with a Pokémon with type disadvantage, or if he loses with a Pokémon with type advantage. Especially potent as of Black and White.
Team Rocket shows up to steal Pikachu using some crazy machine.
Take another if Pikachu is the first one to attack, only for Team Rocket's machine to turn out to be lightningproof this week. (What? Too specific to happen that often? Oh if you only knew)
Take another if they say their motto right in front of Ash and friends, no matter how many times Ash has heard it already.
Finish your drink if they are "blasted off again!" (Or, in Best Wishes, blast themselves off again.)
Are you sure you're not trying to kill us!?
Whoever answers correctly first on "Who's that Pokemon" or "Pokémon Trainer's Choice" doesn't have to drink. Everyone else has to take a sip. Whoever gets it wrong has to take two.
If the Pokémon Trainer's Choice answer is incorrect, everyone takes two sips. If it's the "Arbok evolves into Seviper" one, everyone finishes their drink.
Hell, just drink whenever someone says the name of the Pokemon of the day. If it's a plot important episode, take a shot whenever Ash yells Pikachu's name or if Dawn yells Piplup's.
When Ash's Gym battle is interrupted by Team Rocket
Chug if the gym leader gives Ash the badge right after
Take a shot every time the type matchups are mentioned. (Did you know Grass is weak to Fire? Grass is weak to Fire! Fire-types have an advantage over Grass-types!)
Additionally, take a shot every time they still get it wrong, whether getting the matchup itself wrong or ignoring secondary types (or Levitate in AG and after.) Seriously, even if the Pokémon that are used by whoever's supposed to win were 10 levels higher, some of the outcomes would be incredibly unlikely.
Take a shot each time Meowth falls in love with someone. Take another if he only knows them for one episode and then never mentions them again.
Take two shots when a Pokémon is hit by an attack it is supposed to be immune to. Three shots if it's a poison attack on a Steel-type or an electric attack on a Ground-type. Levitate and similar abilities are ignored pre-AG and the Magnemite family is considered pure Electric in Season 1 for the purposes of this.
If a rival appears and for every douche action he/she does, drink.
Drink 5 bottles if it's Paul, and don't count every action he does. Your kidneys would hate you if you counted them.
When someone makes a blantantly bad move just so that the writers can let the other person win, take a shot.
Two if it's during a gym battle or league match.
When Dawn has a problem with her hair.
And whenever she utters her Catch Phrase: "No need to worry!"
If you encounter a shiny, only for it to flee or faint before you can catch it, finish the bottle. (You'll need to drown your sorrows anyway.) If it faints from Struggle recoil or a critical hit, finish everyone else's shots too. (Roaming Pokémon don't count. The plot-relevant Gyarados and other encounters that are always shiny don't count.)
Take a drink every time the opponent's Pokemon somehow survives with one HP left from a regular, damaging move and the opponent didn't use Endure the turn before. Take another drink if the opponent eats its held berry after surviving.
Take a drink every time the opponent uses Protect or Detect. Take two if the opponent uses the move and gets the protection twice in a row.