Drink once:

• Marty is called a chicken (twice if accompanied by sound effects)
• Biff knocks on someone's head and yells, "Hello, anybody home?"
• Someone is shot at or threatened with a gun (twice if they narrowly escape death, three times if they actually get hit)
• Someone calls someone else a butthead (twice if it's not Biff)
• Someone ogles a woman
• People kiss (twice if they are about to but get interrupted, three times if it's incestuous)
• Someone is hit in the face
• Someone speaks in a foreign language or with an accent (2015 slang counts)
• A Tannen runs afoul of noxious substances
• Doc names something after a prominent historical figure
• Someone expresses surprise, dismay, nostalgia, etc. about some element of the time period they are in
• An actor has multiple roles (twice if two or more of his/her characters are shown onscreen together)
• Doc's experiments don't work (twice if they do)
• Doc exclaims, "Great Scott!"
• Marty proclaims that something is heavy
• Marty does something painful or risky (twice if it involves a car)
• Someone narrowly escapes death or serious injury
• Marty backs down from a challenge (twice if he changes his mind and decides to fight)
• A song from the soundtrack appears in the film
• Someone looks disgruntled
• Marty hesitates
• Someone is insulted
• Clara is shown inside her house (in the doorway or on the porch counts)
• Doc is shown inside his lab
• Someone falls for an obvious joke or gag
• Someone is addressed by their last name
• Someone yells, "Noooooooo!"
• Someone says "future" or "paradox"
• Marty says, "Mom, is that you?"
• Marty looks confused

Drink twice:

• Marty gets the crap beat out of him
• Marty beats the crap out of someone else
• Marty uses a false name
• Clara acts in an unladylike fashion
• Someone is on the verge of panic
• Doc mutters to himself/acts mentally unstable
• Doc and Marty reverse roles--Doc acts like an irresponsible teenager and Marty has to be the sensible adult
• The DeLorean malfunctions
• Someone fades from existence
• Marty is pursued by a Tannen
• Lorraine acts seductive
• George is shown writing something
• A song that's not from the soundtrack appears in the film
• A character quotes something another character has said
• Marty displays his guitar-playing prowess
• A scene from a previous movie is replayed in another (three times if the actors differ from movie to movie)
• An animal is used in an experiment
• Main characters have a spat
• A female character hits or kicks someone
• Someone acts in a sneaky or furtive manner
• Someone or something foreshadows future events
• Someone sees his/her past or future self
• Townsfolk treat Marty like a freak
• Clara gets mad at someone
• Marty's voice goes up an octave
• Marty understands what's going on

Drain it, fill it, drain it again:

• There is gratuitous use of special effects
• Doc goes into a lengthy explanation on how someone's actions have affected the space-time continuum
• Someone does something they ought to know better than to do
• Someone blurts out revealing information about the future
• A popular music artist, producer, etc. has a cameo role
• Someone risks destroying the fabric of the space-time continuum by interacting with their older of younger selves
• Someone's actions change the future
• A scene involves plenty of exposition
• There's a plug for another Universal movie
• Jennifer is rendered unconscious
• George overcomes his wimpish impulses
• The death of a major character is shown or implied (fading from existence counts)
• Doc speaks techno-babble
• An expensive or valuable object is destroyed
• Someone other than Marty or Doc discovers the time machine
• The DeLorean is refueled
• The phrase "back to the future" is used