Answer:
There is no one thing that you can do to ensure admittance to one of these top-tier schools. Because there are so many more qualified applicants than places available, thousands of valedictorians with perfect SAT scores get turned down each year.
The best thing that you can do is to cultivate your own interests and passions. Many people only look at their high school years in terms of what they can do in order to get into an Ivy League school, but in doing so, exhibit no real passion. That passion is what the Ivy Leagues are after, because passion is what breeds invention and change. The admissions committees are looking for people who will make a difference in their field, and will contribute something to the university.
So ask yourself - what is it that I LOVE? What would I love to spend the rest of my life doing? Beyond that, what are the hobbies I truly enjoy, the causes I feel strongly about? Get as involved as possible in your 'thing.' Don't be afraid to be non-traditional - do a foreign-exchange program, or graduate a year early to spend a year immersing yourself in something you love.
In the meantime, learn how to write a KILLER essay. Start looking at your life in terms of short, concise stories, and keep a writing journal so that when the time comes, you'll be able to punch out truly memorable essays. Study hard, even though it's not terribly fun. Read a lot. But don't stress out too much - the beautiful thing about figuring out what you love, and thus, who you are, is that Ivy League designation becomes MUCH less important. Volunteer in an orphanage in Myanmar and you'll realize that whether you go to Princeton or Long Island University, it doesn't matter. It's all about what you do with the resources you have around you.
Good luck! :)