Head inside the 'Carousel Entrance'. For a while it has had scaffolding around it, so most people walk past it and don't think to use this entrance. That's exactly why you should. It's quicker and quieter. When you go down the escalators, there will be a split in the corridor with a sign reading 'Entrance' one way and 'Tickets' the other. Follow the 'Tickets' sign to a small souvenir shop, here you can buy your tickets, quickly and stress free. Then continue on following the signs to the Museum Entrance and you'll breeze right through past all the lines.
There is a food court inside the building, before the entrance of the Museum, so it is more convenient to spend the whole day there.
Now, everyone is visiting to see this painting. No question, if you have even 5 minutes in this building, that's all you'll look at. Which means crowds.
Crowds bring 2 things - problematic photography environments and pick pockets.
My advice - If you have a good enough camera, try and take your money shot from the back of the room. If not, you just simply have to put your big boy pants on and shove yourself to the front of the crowds. Once at the front of this crowd, taking your insta-selfie, don't forget to look at the painting to which the lady herself is staring at. It makes you think maybe why she has such a cute little snicker on her face.
Also in this Museum, but particularly this room, be aware of pick pockets. You will have many hands and bodies pushing against you, all while you try and focus on the right exposure for Her Lady. Zip up your bags and don't keep valuables in your pockets.
Ladies, this one's for you. Throughout the museum, there are air vents that blow cool air into the building, due to all of the people inside. This is great, but they are located on the floor....
I wore a dress to The Louvre, and with so many people and amazing artwork around, I wasn't exactly watching where I was walking.
Many Marilyn Monroe moments were had, and they weren't flattering. So either wear pants, or just be aware of where you're stepping.
Last piece of advice, don't forget to look up!