Im a Sprinter in Track & people tell me that I run flatfooted...what exercises should I do to prevent this?

The way you run often depends on your shoes. So that may be one way to change your stride.There arent any exercises for it, you just need to start running the right way untill it becomes a habit. Also, I'm not a sprinter but I am pretty sure you are supposed to be running toes first, not heels.