This is version 1.0. When I think of anything else I'll revise it. Feel free to leave suggestions in the comments.
Navigate to the subtitles menu. Take one drink for each language the disc offers.
Boy Meets Girl
Drink if she's engaged. Twice if it's arranged.
Drink if he's got some hyper-masculine life-threatening job, like helicopter pilot or racecar driver.
Drink if she puts him in his place via unexpected athletic superiority. Twice if both actors visibly suck at the sport in question.
Drink if he pulls a bafflingly insensitive practical joke. Do a shot if it involves faked death.
The Musical Numbers
Drink for each costume change. All of them.
Drink for each gratuitous panoramic shot of the beautiful Indian countryside. Twice if they were nowhere near this mysterious lush valley when the song started.
Drink if someone picks up an instrument and pretends to play it. Twice if they're not even close to being in synch. Thrice if they look like they're not even sure how to hold it. Jäger shot if the instrument in question isn't audible on the soundtrack.
The Language Barrier
Drink whenever there's a grammatical error in the subtitles.
Drink if you hear an English phrase amidst the dialogue, like "How's it going" or "Excuse me" (doesn't count if the whole scene's in English).
Waterfall (see def. 2; def. 1 might come in handy as well)
Start drinking when the comic relief character's one joke gets really old. Stop drinking when the scene ends or you finish your beer, whichever comes first.
If anyone laughs during a serious scene, they have to drink.
Drink if you recognise an actor from another movie. Everyone else drinks if you can remember the title. (NOTE: This counts for the stars, too, so as soon as someone shouts out the name of another Shahrukh Khan movie the first time he appears on screen, everyone else has to drink.)
Chug one at the intermission.
And do a shot when you break the three-hour mark.