From memory I believe there was three important rules for breaking up, the first being you should never put off breaking up if you know you want to. Prolonging the situation only makes it worse.
Tell them honestly, simply, kindly, but firmly. Don't make a big production or an elaborate story - this will help avoid a big tear jerking scene. Even if you've only been together for a short time and haven't been to serious. There's still some feeling of rejection when someone tells you they prefer the company of others to your exclusive company.
If you're honest and direct, and avoid a flowery emotional speech when you break the news they'll respect you for your frankness and honestly appreciate the kind of straight forward manner in which you told them your decision. Unless they're a real jerk or cry baby, you'll remain friends.
Oh yeah, and if you see Johnny football hero in the hall tell him he played a great game, tell him you liked his article in the newspaper.