Photocopy a spare orderform, change the date on the top to suit the year and send it off 18 months in advance. There may be other methods for the computer literate but I have been using the same photcopies since 1999 and it has been accepted every time, as long as you remember to change the date.
I think it was Smokie who mentioned something about a preferential customer list but I don't know how you go about getting on it as I have never failed to get tickets by using the tried and tested photocopy method.