I am going to work on my first brevet series this year, so I don't yet have good suggestions for training. But there are a couple of things I would recommend.
1) Join RUSA (
http://www.rusa.org). Once you join, you will get the Member's Handbook, which is a 140 page goldmine of information on the sport of randonneuring.
2) Check your local brevet schedule. As far as I can tell, to register for BMB, you need a full brevet series (200k, 300k, 400k & 600k) in 2006, which really means by the end of July. Make sure that there are enough brevets scheduled between May and July in your corner of the world, to be able to qualify for BMB. Here in the Northwest, the brevet series start in late March and end mid June, and starting in May would not be possible.
Based on my limited experience (three 200k brevets) and what I heard from other randonneurs, each level of brevets requires different physical and mental skills. Make sure to train your mind (and your stomach) as well as your legs.