I've found the easiest way to order tickets for the following year is to E Mail
ticket@lemans.org with the details of requirements making sure to highlight the year for which they are required IE
2006, in the fullness of time a confirmation letter is recieved and then as a Christmas Present the ACO send you the bill which you have to pay in 2 weeks (just when life is so expensive), once paid just look forward to June.
Of course don't forget the ferry booking.