A long throw-in can be a greatly used tool for any soccer team.
#1 Is obvious... Practice your throw. Get comfortable with it. Every time you go to perform a long throw-in you will know your distance and wont have to think about the technique. You'll be able to focus more on hitting the target area.
#2 You need to try to relax your body while doing it so you don't restrict your motion.
Great question.....Throw-ins can be a very vital part of your teams set plays. A throw-in down towards the opposing teams goal can easily turn into great scoring opportunity. So to answer your question...
#1. contrary to popular belief distance does NOT come from the strongest person on your team.
#2. your distance is mostly going to come from form.
#3. difficult to explain through words; however while taking a throw-in if you could picture your body in the shape of a backwards C, obviously with your feet on the ground. You want to have your lower back arch as much as you can without causing pain. This will give you more leverage.
#4. flexibility will be a huge part of getting your back to arch properly and more efficiently.
#5. also your run up to the sideline and your timing will be a huge part of your efficiency to throw the ball in with great distance!
For Chris and others still interested. Long throws come from core strength and technique, not arm strength. To practice, find a sturdy wall, start close and throw against the wall. Challenge yourself to have it bounce back over your head. Each time you do, take a step back and challenge yourself again. Keep track of your mark for the next time. Practice, practice, practice.