Train with weights three days a week, with a day in between to let the muscles recover. Focus on the leg and shoulder muscles, but don't ignore anything.
Train with a medicine ball on the days without weight training. Working with a medicine ball helps with explosive movement, increasing the distance a javelin will travel. Hold the medicine ball and perform lunges, lunge crossovers and leg chops. Practice sprints with the medicine ball held overhead. Find six to eight comfortable exercises, and do three sets of 8 to 10 repetitions.
Practice sprints three or four days a week. The run before a javelin launch is 13 to 17 steps and the faster the run is the better. Practice 30, 50 and 100 yard sprints.
Work on form with a partner, preferably someone with experience throwing a javelin. Practice starts and throws while the partner watches with a critical eye. It's very difficult for someone to judge their own form, so a partner paying close attention can see mistakes that you might miss.
Accelerate past the javelin before throwing it rather than pulling an arm back to throw it. Pulling the arm and shoulder back reduces the throwers momentum, but an explosive burst of speed brings the hips forward so the arm and shoulder fall back naturally.
Release the javelin at a 33 degree angle to the ground. This is the optimum angle based on the javelin's aerodynamic lift and drag.
Rest one day a week. Practice the other days, but use one day to let the body recover. Athletes who don't rest occasionally will burn out, which leads to reduced performance.