I want to become flexible enough to do the splits and other such feats of flexibility. How?

I am a 17 year old boy, 5'10, 160 pounds, in good shape. I swim and play rugby so I consistently exercise, but I am not as flexible as I would like. First of all, how can I increase flexibility to the point of being able to do the splits and have good range of motion? Secondly, would it be detrimental for my performance in athletics? I am willing to work hard.