I was a semi-finalist this year but did not advance as a finalist. However, I did speak with someone a few days ago who was a finalist about their experience.
My biggest advice would be to talk in depth to semi-finalists and especially finalists and get a feel for what they emphasized on their applications.
Also, this past weekend when all the finalists were in Atlanta I heard that the president (president of Coke..not the US haha) or someone talked to them about how they were selected. The girl I talked to said that, for example...there were 84,000 kids that applied. If 83,000 of them are on student government, then that has less of a weight/importance. But if only 1000 were on student government, then it's a bigger deal. So my analysis of that means they're looking for kids with really unique attributes/service resume.
I would just do EVERYTHING you possibly can this summer EC and service wise. Build up your resume and put down EVERYTHING you've done.
Also, service isn't everything. I think they're definitely looking for someone that comes across as well-rounded. I heard that most of the finalists (and the other semi-finalists I've heard of) were all involved with a sport, music, academics, had at least a summer job, and were just overall sucessful and active in their community. Obviously, that's the not the case for every semifinalist or finalist, but being well-rounded definitely helps not only for this but also for college admissions and other scholarships.
On the original application just to give you heads up, it'll ask you how many service hours you did each year.
Best of luck! It was an honor being a semi-finalist for this, but man, I wish I could have been a finalist! I've been blessed with other scholarships though so I'm definitely lucky in that way.
And another scholarship that seems like you'd be into is the Elks MVS. My deadline for that was December 1st or 2nd, but other lodges had their own deadlines so make sure you check that out. Build yourself a portfolio/application and include pictures of yourself doing your Habitat stuff in the section where you can include pictures of yourself. That definitely seems like you'd be good for that. Best of luck!