To those saying "look at Will Shoestring" "look at Paige Pierce" and "look at that 13 year old twig" - That is completely beside the point. The point is: YOUR NAME is not Will, or Paige, or Anthony Jr. And if it is - well, stop correcting me - the point is: You can't throw as far as they can naturally. Great form is completely and totally necessary for distance throwing. Without great form, you're not going to throw far. I promise.
But: you CAN enhance the distance you achieve through weight training. I'm just some 5'10 guy. I've never done anything remotely impressive athletically. I'm not a physical specimen. Even with proper form there's no reason for me to be throwing way further than a SLEW of other local Ams and Pros in my area. But amongst the regular guys in my area (discounting a couple of guys who no longer throw)? I have by far the longest distance around here.
The reason for that? I spend every single winter dedicating myself to working out and lifting weights. And it is NOT arm work that does it. It is full body core conditioning - from the legs on up. There is no single workout - but as long as you've worked with a professional to design a workout plan, you can and will improve your distance if you strengthen your body.