This grand museum is really huge, with no exaggeration. Guides will lead you along a fixed route featuring famous works such as the Mona Lisa, The Last Supper, the statues of Venus de Milo, Nika, David, and many other masterpieces. Having a guide is really invaluable during the peak season when you just can't do without the accompaniment and it's quite difficult to pave your way through the endless tourist flow.
However, they will show you only a small percentage of the world heritage stored at the Louvre. They likely won't take you to the Egyptian expo, or other less-known but still mesmerizing hidden corners of this museum. The best time to explore the Louvre on your own, and at your own pace is at night when the visitors are few. The best season is winter, again owing to short lines.
From October to March, you can go for free on the first Sunday of each month. Free admission also means no queues for tickets, thus you can enter faster; yet, it also means more people inside.
To avoid long lines, approach the Louvre from Rue de Rivoli and the underground shopping arcade (Carrousel de Louvre); there is a second entrance here that is far less busy on any day. Obviously, we suggest not to enter from Tuileries (the famous pyramid), where the tourists usually queue at the entrance.
Evenings are usually less crowded, especially on the late opening days: Wednesdays & Fridays the museum is open until 9:45 p.m., so you should take advantage of the of the operating hours.