The best advice is to try to work in a few short weekend tours before the big one. Let your body get used to hauling all that gear on the bike, and while you're there it will give you an idea of the things you'll need the most, and the things you can do without when you're out there on the road. I'd say it's also important to get outside as soon as you can, so you get some practice at dealing with the conditions you might face on the road. If you're riding a long tour, it's probably a given that it *will* rain at some point, and your tour will be a lot more enjoyable if you're comfortable with things like that.