Go to the ACO website now, find the order form for 2008, print it, tippex out 2008 and amend to 2009, complete it with your first three choices of campsite then email or fax it back to the ACO. The ACO will throw it in a box and in December will allocate you tickets for BSJ or Bleu Nord, they send you notification of the allocation and you chose whether to except them or not.
If you have someone who has ordered tickets previously put the order in their name as the ACO seem to work in some kind of priority based on the account number of the person ordering - the longer you've ordered the better chance of getting what you want.
Oh BTW, our French campsite neighbours tell us that they already have their allocation for MB for next year and they've only been going for the last three years...so if you speak French and have a French address to use I'd try ringing.