Practice, practice, practice. There are some who say weight training will pay dividends, but the most proven ways to improve your distance is “timing, technique and release”, as Mark Jennings says. When the entire symmetry comes together, you will get a hold of that one throw that you know is just going to GO. That being said, as in any sport, speed = distance. NOT power = distance as commonly thought. With a golf ball, it isn’t if you crush the ball. It is the speed of the club head. So concentrate on a faster arm swing through the motion of the throw. Not trying to throw it hard, but trying to whip your arm around fast.