Society at large is made up of norms and certain forms of behaviour, and while it's pretty punk rock to break rules in general, being an asshole in the process should not be a pre-requisite. There are simple things in day to day life that are pretty important to do, such as offering your guest some water or being quiet in the movie hall, that are so commonplace they're basically unspoken. However, it's nice to be reminded of things sometimes, especially when it comes to positive social conduct.
Here are some unspoken rules of social etiquette.
1. Returning borrowed money
Don't delay giving that borrowed cash back to people, especially people you don't know that well. I myself tend to follow an installment system, but I wouldn't recommend it.
2. Flushing at someone else's house (or anywhere for that matter)
Never forget to flush, and if the water is over, then suck it up and confess. You could also try the ' baalti technique', where you just overload the toilet mainframe with a high velocity bucket of water.
3. Being quiet in the movie hall
Don't ruin the movie experience for everyone around you by screaming on the phone or clapping/ whistling every time your favourite b-grade action hero shoots down a helicopter. Stay classy.
4. Calling before showing up at someone's place
This can be extremely annoying for the person who has to open the door. A simple call or text to confirm can help you avoid being an unwanted guest at a person's home, unless it's your best friend however, in which case anything goes.
5. Not showing people the finger in a traffic jam
We're all in the same boat. Honking needlessly and showing the other beached whales the bird isn't exactly going to help matters. Listen to one of those 'Be Zen' audio tracks instead.
6. Not getting too drunk on a flight
Usually I'd complain about how they don't offer alcohol on most domestic flights in India, but after seeing a couple of people literally lose their shit on international flights, I'm not that mad. Have a drink, have a good time, go to sleep, just don't start pissing on other passengers!
7. Letting people get off the metro before getting in
This is more common sense than anything else. In fact, it doesn't even need to be explained. Let the people out before you get in, and everyone gets what they want.
8. Offering your seat to disabled or elderly people in the bus/metro
If the person looks like they really need a seat, give it to them. You'll feel better about helping out your fellow man.
9. Offering someone water when they come to your house
In some nomadic cultures, a guest will not ask for water from the host even if their throats are parched. Offering water to someone coming into your home is a basic thing to do. You could also offer them Tang, I never say no to Tang!
10. Responding to an event invite
It doesn't take too long, and it might not always be a big deal, but not letting a friend know whether you're going to their party or dinner or whatever may just tick them the wrong way.
11. Being punctual
Whether it's in an office setting or simply being late to a movie, they're all causes of annoyance. Being too early is also a thing however, so try to gauge the situation right.
12. Sticking to your position in a line or Queue
Whether it's at a Barista or a public office, breaking queue to get to the front of the line is basic blasphemy. Don't be a douche, stick your place in the line, and avoid getting your ass kicked sooner or later.