We were in Paris recently, and never found the Louvre's "red awning" entrance for groups and museum-pass holders that Rick recommends; but we waited only 7-8 minutes at the Lion Gate entrance, as opposed to a serious mob scene at the pyramid.
To find it, stand with your back to the pyramid entrance. The Lion Gate entrance is to the left of the small triumphal arch that is across the big street. Look for the lions :-) I had read that tickets are not sold at the entrance, but they were being sold when we visited.