Increase your jumping ability by getting in better overall physical shape
Being able to jump high is the result of a combination of things: natural ability, leg strength, explosive power, and your overall level of fitness. One way to get in shape, and stay in shape, is to play lots of basketball.

If you play a lot of ball, that will tranlate into being in better shape. There's no way your vertical leap will improve if you aren't in good shape. Also, there are lots of quick movements you make in a game that you can't recreate outside of actual game play. These help build your overall basketball abilities, as well as your athleticism (explosiveness, jumping ability, etc.).

Jump rope exercises
It's an old school exercise, but it works. Jumping rope is a great way to build stamina, and work on your leg strength. In addition, it builds jumping ability (explosiveness, calf strength, etc.). Adding jump rope exercises to your workout routine can pay big dividends in increasing your vertical leaping ability.

Run stadium stairs
Another old school exericse, and a great one for helping you develop length strenght and power that will help you become a better jumper.

Running stadium stairs can really help your vertical leap because it helps build stamina, leg strength, and when done right, explosiveness. To work on explosiveness, use "bounding" movements. In otherwords, don't just get from one step the the next, but really leap up from one step to the next.

Wall sits and squats
Strength training (like wall sits and squats) will help you build leg strength, which is a key component of athletic fitness, and therefore jumping ability. Remember to only use a weight that is comfortable for you, and to always have a trainer or training partner work out with you. You don't want to get hurt lifting too much weight, or lifting it improperly.

Explosive leaps
This is a great type of exercise. Do these at a park, or on a football field so that you are cushioned by the soft grass surface.

One-legged jumps are done like this: leap as high as possible off of one foot, then come down and immediately leap as high as possible off the other leg. Repeat this for 10 to 15 jumps.

Two-legged leaps are done like this: squat down so you are in the triple threat position. Now explode up and off the ground as if you are reaching for a rebound. Come down, gather yourself for a second or two, then leap up again, as high as possible. Do this 8 to 10 times.

Remember, vertical leap is part leg strength and part explosiveness. In fact, the explosiveness part is the more important of the two. It's not about the size of your leg muscles, or how much weight you can put up in the gym. It's about your athletic ability, coordination, and your ability to explode up and off the ground.