Every time someone drinks anything
Every time a Hobbit talks about food.
If the Hobbit is complaining about being hungry
Every time Aragorn and Boromir have an argument
Every time someone looks all mopey and angst-ridden
Every time a Black Rider appears
Every time someone sings a song
Every time someone draws a sword
Every time Frodo's eyes roll back into his head
Every time someone or something is called "precious," take a shot.
Every time you see the Great Eye
Every time Elrond appears onscreen
Every time someone uses the word "Mordor,"
There's a close-up of the ring
Merry and/or Pippin do/say something stupid
Legolas says something poetic or looks off into the distance
Gandalf says something wise/mysterious
Someone speaks about the Strength of Men or Aragorn's bloodline/future.
Out of respect, when a character of importance passes away
Whenever Frodo falls over.
Whenever there is an Elf vs. Dwarf moment.