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.