Every time the cold water is a little too contrived.
Two sips if it's the Tendo Koi Pond.
Two sips for convenient rain.
Extreme Rule: Every time someone with a Jusenkyo Curse changes. The sad part is, this one won't kill you past season 3, if you're watching anime.
Every time Akane says "baka"/"jerk".
Every time somebody uses a Megaton Punch (or kick).
Two sips if it's Akane.
Two if it's on Kuno (does that make three or four if it's Akane on Kuno?).
Every time a battle scene is resolved with Hit Flash into a Fight Unscene. This one starts slow, but...
Extreme Rule: Every time Hit Flash is used. This one will kill you; don't be fooled by the slow start.
Every time Happosai gets Hoist by His Own Petard.
Every time Happosai says, "what a haul!".
Every time the Happo Fire Burst is used.
Every time a named attack is used.
Two if it's an ultimate attack which...isn't.
Every time Ranma gets caught being an Accidental Pervert.
Two sips if it's Akane who catches him in the act
Every time Genma or Soun cry.
Every time Genma pretends to be an innocent panda.
Every time Girl!Ranma's assets are on show.
Every time Pig!Ryoga gets Marshmallow Heaven.
Every time someone says "An opening!"
Two sips if said opening turns out to be no help at all.