I was just about to post the same question, Ive been going to the Gym pretty much 4/5 times every week - mainly doing cardio excersise's for 2 hours. Just looking on the internet, and found information pretty much the same given here ie. Work on your leg muscles.
You can buy those leg weights which Hurdle Jumpers use to increase the load onto your legs, but you would look pretty stupid on a paintball field.
Ripped from Men's Health - England Rugby Tips...
Jumps to Box
1. Position a sturdy box, or platform within jumping distance. The higher the box the more explosive the jump
2. Use your arms to lead the initial fast decent to a full squat position, then swing rapidly to upward movement.
3. Spring as high as you can on the box. Step down and repeat.
Lateral Jumps (good for explosive change of direction)
1. Set your feet shoulder-width apart. Repeatedly jump over an obstacle (sideways).
2. The key is to spend the minimum time in contact with the ground and generate as much push-off force as possible.
Hope that helps?