Know the difference between war and peace
There are certain, particular very crowded situations where decorum is expected, and others where it is definitively not. A few examples:
At the museums, it’s peace
If no one else is viewing an exhibit, stand directly center and take that masterpiece in. But the second someone joins you, step to the side to flank it. Sure, you’ve waited all your life to be this close to Christina’s World, but so have the 20 people next to you.
At the grocery, it’s war
At Trader Joe’s, “Full Contact” Fairway, and other great groceries with good prices, the accepted norm is to act as if the aforementioned apocalypse is coming, and you’re fighting to stock up your shelter. So if an old woman snatches a cottage cheese out of your hand because it’s the last one, don’t berate her, just tell her “well played” and reconsider your dairy choices.
It is also war at sample sales
From a tipster: “The etiquette for sample sales in New York is to realize that all etiquette goes out the window the second you step inside some huge loft or warehouse space, packed to the gills with discounted designer duds, because New Yorkers turn into feral animals inside those four walls. Accept the fact that you will be shoved, your feet will be stepped on, and that you'll have to fight for that glorious blouse (one size too big, but whatever, you'll make it work!). Instead of raging internally about how horribly everyone is behaving, come prepared for battle instead. Wear shoes that are easily removed, a tank top that you can try items over since there likely won't be dressing rooms, and whatever you do, don't make eye contact with your fellow shoppers -- it will only remind you that the person holding the other strap of that leather bag is a human being, your defenses will be worn down, and all of the sudden you'll hear yourself saying, ‘No, you take it, I'm sure I can find something else.’”