I first watched the show a few years ago when David Cook won it. I was really interested in it that year. This year, my wife and I hardly watched it after the top 12. I think Simon's getting out while the show's on top. I don't know...
But, based on the last three years, here is how you win.
1. Be a guy. While both genders watch the show, teenage girls are the major voting bloc. All else being equal, the cute guy gets more votes than any girl.
2. Don't start out too strong. David Archuleta, Adam Lambert, and Crystal Bowersox were the early season favorites the last three years. They all came in 2nd. I don't know about prior seasons, but I know Jordin Sparks was not the early season favorite her year.
3. Play the guitar. Since instruments have been allowed, the winner each year has played the guitar. I think it allows for more diversity in the performances (to keep you "fresh" for longer), and is also perhaps a sign of musicianship.
4. After that, just the other stuff - you know, sing well, don't be a punk, look somewhat attractive. Those things.