It’s really not that hard. But it’s always appreciated and always appropriate. It can be something as little as saying “thank you” or asking someone else how they are (even after you’ve already said how you are doing). Sometimes being polite just takes holding a door open on your way out.
Unfortunately not everyone realizes that being nice is really that simple.
Case in point:
I was entering the library. A man was leaving. A woman with a large stroller was right behind him, trying to negotiate the two doors before the large number of steps to the sidewalk. The woman said “Excuse me” and asked the man to help. The man stopped. He was still close enough to open the door. Then he looked at me, told the woman that I could help and kept walking. I said “Charming” really loudly in a sarcastic tone to his departing form. The mother offered similar sentiments.
But she had nothing but nice things to say to me, so really, it was worth it. Everything you put forth into the world will come back to you after all.