You are waiting in a slow line. When another line opens up, the people behind you rush to get there first.
Unless the cashier motioned to the people in line behind you, you can politely say "I'm sorry, but I was waiting in line before you, would you mind if I go first?" If they say yes, great. If not, you should probably drop it, because you are clearly dealing with people are not going to be sensitive to the needs of other.
One of your coworkers regularly talks badly about a workmate she doesn't like with a group in the office lunchroom.
If you are merely eavesdropping, you don't have any grounds to comment. If you genuinely couldn't help but overhear, you can suggest "Tracy, I don't mean to overstep but if you are going to speak ill of Brenda, don't you think you should do it somewhere more private? She could easily walk in on you in here." If what she is saying is blatantly false, you can also point this out to her.
Out and About:
As you walk down the street, you see a man and his son throw empty paper cups to the ground. There is a trash bin no less than 10 feet away.
Don't make a fuss and glare at the litterers, merely lead by example. Pick up the cups and deposit them in the trash.
sourced from Emily Post's Etiquette