It's not that hard, and after a few weeks out of the city, I have realized that while most of the time people are very lovely, sometimes they forget their manners.
My message is a simple one: Be Nice.
If someone bumps into you accidentally, it doesn't matter. If they apologize, give them a friendly nod to let them know you accept their apology.
When you're on the subway, it sucks when the carriage is full and you're all squishy. You can still be friendly about it though and try and do your best to not inconvenience others around you.
If you walk into a store, say “hello,” to the person working there, and say “goodbye!” when you leave. It's not hard but it's friendly.
Same goes for when you go to the supermarket. You don't have to talk to them about their life story, but you can be nice and smile and say “hello!”
Someone tries to engage in conversation with you randomly? This is a lonely city if you don't have friends. If you don't want to be their friend, be polite about it!
Reach out to your friends, make sure they're doing ok.
High five a stranger.
Keep a smile on your face when you're walking.
You'll be surprised how much nicer you can make this upcoming winter for you and everyone around you.