My advice to prepare for the application process:
Keep your grades up! Sign up for the toughest classes you can your senior year as well, not only to prove that you're pushing yourself, but to prepare you for the academic rigors of the academy.
If you haven't already, familiarize yourself with your guidance counselor and whoever will be sending out your transcripts (at my school they're separate). Find out how the teacher recommendations at your school work. Do they turn in the rec's to you or to the person who sends out your transcripts? Will your teachers be okay with writing general recs that could be applied to any school, or do they prefer having a specific college/MOC to write to?
Start writing a resumé, so you can ensure you have all of your ECs, accomplishments, et cetera on them. Make sure that your strongest points stand out on the resumé. It's helpful to hand resumés in to your teachers when you ask for recommendations, because it helps them get to know who you are outside of their class.
Look into backup schools. I honestly had my heart set on West Point by the beginning of my junior year, and had no idea where I'd want to go if I didn't get in. Look to apply to about five schools (that's what I did, anyway). If you can narrow them down now, that's wonderful (I didn't decide until just a month ago) -- it will give you time to visit them over the next year, and you will also be able to ask your teachers for specific recommendations. Know what each of those schools needs for admission, and write it down to help keep track. Do you need SAT Subject Tests? Sign up to take those. Do they accept AP credits? Remember, ANYTHING could happen in the next 17 months, and you NEED a plan B.
Get in shape. Start a workout plan now, and set incremental goals with an accompanying long-term goal. Stick to it.
That's basically all you can do now. The hardest part of the application process is all the waiting, waiting, waiting.
But, in the future:
Before you get out of school, contact the teachers from whom you want to get the evaluations and recommendations for senators and other schools. Let them know you will be sending them an email in early August, most likely, when the Candidate Portal comes online, and find out an email where they can be reached. It would also be a good idea to get recs from them in late spring, when they won't have tons of your classmates also asking them for recommendations.
If you can, find a writing teacher with whom you're close, and ask if s/he'd be available to revise a few essays in early-mid August. If they can, great, and if not, that's okay. As soon as your three essay prompts are available, start working on them. Aim to have them finished by the time school starts, or earlier if your teacher is available for revisions.
AS SOON as it's available, SCHEDULE your DoDMERB. Many things can go wrong with the medical stuff -- I was disqualified for a history of a blood condition that I haven't been affected by since I was three. It took me six months to resolve the case -- if you wait to schedule your medical examination until January and it takes you six months to resolve the issue, you might be SOL.
Stay busy over the summer! Many summer activities can boost your resumé. I also found it's also much easier to volunteer over the break. If you can, attend other college summer programs as well.
Know what your MOC's require for nominations. Don't turn in your request too early, as some others noted, because you may receive other awards or positions that can boost your chances. At the same time, don't wait until the last minute. Personally, I would aim to have it turned in by 3-4 weeks before the due date, which would leave more than enough time for slow mail.
Wow, that was a long post. Hopefully it helps. :]