1. Get your financial and other affairs in order. You won't have a chance to tend to them for a while. Have mom or dad (NOT YOUR GIRLFRIEND) tend to your accounts while you're gone. Have a special power of attorney drafted so they can sign for you, if necessary. If you do have a special someone in your life, don't rush off and marry her before you go. Trust me, it usually doesn't turn out the way you think it will. (Disregard the above if you're already hitched.)
2. Stretch every morning for about 20 minutes, starting with your neck and finishing with your calves/ankle area. Don't worry about trying to get a head start on what you think the physical program will be since 12 days isn't enough time to accomplish much. Just stretch.
3. Enjoy your favorite foods, music, movies, magazines, books, etc. Spend quality time with the people you love, but whatever you do, don't go into the "if anything should happen to me" speech.
That about sums it up. By the way, who did you join, and what will you be doing?
edit: Oh yeah, one more thing: No matter what happens, no matter what anyone says or does to you, no matter where you find yourself or what you find yourself doing...you volunteered!
Good luck!