A recent article in the Journal of Strength and Conditioning Research (Volume 23, no. 2, 2009, pages 495-506) analyzed the combined results of 56 studies of plyometric (bounce) training to determine what exercises were most effective in improving jump height. Increases in the neighborhood of 7% were observed after good plyometric training programs. The combination of squat jumps (jumping up from a static squatting position), countermovement jumps (starting from a standing position, then quickly dipping into a bent-knee position before jumping up) and drop jumps (jumping down to the ground from a box, then jumping up) provided the most improvement in jump height. Adding weight to the body using a vest or other means was not better than jumping with bodyweight alone. Also, weight training in combination with plometric exercise did not result in more improvement than plyometrics alone. Programs of 10 or more weeks were more effective than shorter programs. Programs of more than 20 total sessions of 50+ jumps per session were better than shorter programs of lower volume. Box height (e.g. 6" vs 12") for drop jumps didn't matter. The most experienced athletes improved the most. Improvement was seen even in subjects of low initial fitness level.
A word of caution is in order. Most of the subjects in these studies were of college age and many had been physically active before their studies. Plyometric exercise can result in high forces on muscles and tendons, and injury risk is higher than with slow, steady exercises. Plyometric programs should only begin at a very low level, particularly for those people who have been relatively inactive and those above 35 years of age. More gentle plyometric exercises for the beginner include gentle, submaximal hops and jumps. Running itself is a plyometric exercise, especially when running faster in short intervals interspersed with walking or jogging. Tendons and ligaments take a very long time to build up strength, much more than muscles do, so patience is the key word. The intensity and volume of plyometric workouts should only be increased very gradually, and more advanced exercises, like drop jumps, should only be added after a relatively high jump-strength level is attained.