@Consultant is right, if you can, find someone to teach proper form (I personally would look for someone with track and field sprinting experience and not someone that does "speed" training). My son has been working on this a lot recently and it's definitely made a difference. He had hit a plateau "speed" training on his own, but when he started working specifically on form his times started dropping again. He hasn't hit another plateau yet, so I'm excited to see how much lower it will continue to drop.
In the mean time. I would recommend plyometric exercises specifically for sprinters (Google it). Video from the side when sprinting to self-evaluate (there's plenty of good youtube videos about proper sprinting form). Just get out there and start sprinting. No need to run a mile.