Winter weather often chases people in need of some sort of escape to museums. So a refresher on Perfect Propriety at museums might not come amiss.
Don't bring a lot of stuff. Aside from the fact that it's time-consuming to check and retrieve personal belongings, many museums either don't allow backpacks in their galleries, or will require you to wear them in front like a baby in a snuggly. Keep it simple and travel light.
Do bring your member card (if you're a member). Yes, the staff can look up your membership, but why put them to all that trouble, and why hold up everyone behind you in line?
Don't touch the art! Etiquetteer should not have to say this. Keep your hands to yourself. You don't want to cause an incident like these folks.
This also includes sitting on the art. It doesn't matter how tired you are, you can't just sit down on some ancient stone slab carved with cunieform or something.
Be responsible for your children, and don't let them touch the art. Again, Etiquetteer should not have to be saying this.
It's not church, but keep it down. Your voice has the power to harsh the buzz of everyone in the gallery. By all means chat with your friends, but quietly.
Your need to photograph the art is less important than the need of others to view the art. Be aware of where you (and your camera) are in relation to others.
Etiquetteer enjoyed watching crowds of art lovers at the Louvre seeing the Mona Lisa.
Obey photo rules. Most (but not all) museums allow photographs without flash. Know what's Perfectly Proper before you whip out your picture-taker, and be sure to disable your flash. (Etiquetteer's greatest museum fear is that a photo flash will accidentally go off, and a priceless painting will flake bit by bit to the floor.)
Watch out with that selfie, whether you're using a selfie stick or not. Some museums will not allow selfie sticks for exactly this reason.
Be courteous to the staff. Many of them are working there for love and peanuts, or they might even be volunteers. Speak pleasantly, use the Magic Words, and recognize their common humanity.
Etiquetteer wishes you many Perfectly Proper museum visits this winter!