try doing some plyometrics. Here is a website with a workout www.itfnz.org.nz-ref-essays-Plyometrics.pdf
you must have adobe acrobat for it to work though. It might take about 10 or 15 seconds on a dial up connection. Remember to put maximum energy each time you do the excercise. do 3 sets of 10-15 on each exercise. Also an exercise to improve calf mucles is hold one of your feet and hop with the other slight bending your knee and just pushing off with your foot extending.The plyometrics should give you more explosive power. You might also want to train your calf muscles more for improved jump.