Here are some helpful tips we found at the Louvre. We followed Rick's advice and purchased our tickets at the Tabac shop in the underground mall. Very easy with no lines. We did find the price has increased to $15 for a one-day ticket, but also discovered that kids, under 18, are free!
We found the hidden entrance at Porte Des Lions, on the west end of the Denon wing. The lines at the other entrances were long, at Porte Des Lions, there was no line at all! This entrance puts you in the museum a little backwards, and you will need to walk about 10 minutes to the center, to start Rick's audio guide, but well worth the effort.