Hi Bob,
They generally open up the online ticket sales 3 months in advance at 8:00 a.m. Paris time. I can tell you that I was online at 12:00 sharp (opening time in my timezone) to buy the tickets for our August visit and they were gone in seconds - I believe that tour companies and other 'bulk' buyers are buying them up, and the average person has a pretty hard time getting them for peak times. Perhaps it isn't as much of a problem in the offseason? Anyways, we ended up buying a 'tour' from Easy Pass Tours in order to get entrance at the time and day we wanted and skip the lines. The 'tour' consists of meeting at the tour office very near the Eiffel tower, walking over with a guide who gives you a bit of information and history, and then walks you up to the elevator for you to skip the lines. It cost more than the tickets by themselves of course, but it was worth it to us to bypass the 2.5 hour line (this was a couple of weeks ago). It may not be worth it to everyone, but we hate lines! Have a great time in Paris!