Even if you've been jumping since you were a kid, you need to learn the proper technique. Landing improperly can wreck your knees and eventually put you out of commission. You can learn the basics of jumping vertically and jumping horizontally, as well as some good tips for improving your hops. If you're interested in more specifics types of jumping, you can then read up on how to hurdle, increase your vertical jump or even jump onto a wall.
Take one or two preliminary steps. Even if you're going to jump straight up into the air, adding a couple of lateral steps into your jumping routine can help you put some extra hop in your step. The energy developed in those steps can help generate additional upward lift that can put an extra inch or two into a vertical leap.
A vertical jump is highest off two feet. Use the power of both your legs to push off the ground, even if you're taking a few steps before you hop.
Drop into an imaginary chair. To get the most power out of your legs and the most height into your jump, you need to bend your knees. For many people, it's helpful to imagine sitting in an imaginary chair, just before you jump. Your feet should be about shoulder-width apart, and your hips should be flexed at 30 degrees, knees bent 60 degrees, ankles flexed 25 degrees in order to generate the most power without injuring your knees. You should be able to lift your toes up and down while sitting in this squat form, balancing on the balls of your feet.
Be careful that your knees don't point inwards in a "knock-knee" position, with your toes pointed inward. Keep your knees as straight as possible, ideally positioned vertically over your toes. Have your arms at your sides.
Keep your back very straight while you're jumping. Practice in front of a mirror dropping into the imaginary chair and keeping your back straight to avoid injury.