Take a shot every time Kenshin says ORO!
Take two if it was caused by Sanosuke.
Three if it was caused by Okita.
Take a shot every time Okita flirts with somebody.
Take two if that somebody is Captain Jack Harkness.
Take a shot every time Okita tries to kill provoke Kenshin.
Take two if it, somehow, involves pens.
Take a shot every time the twins switch identities.
Take two if Light calls them out on it.
Take a shot every time that somebody has a nosebleed.
Two if it's a Bleach character.
Three if it's Renji.

EDIT!
Take a shot every time Valyn facepalms.
Two if it was caused by Shadow.
Three if it was Keman.
Finish the bottle if it was Shana.