Repetition is the key to improving your vertical. Focusing on strength and explosion are the second keys. Jump as much as possible. The muscles need to be trained for explosive movements. Squats simulate the first stage of jumping which is the preparatory stage. This exercise builds strength in the quad, glutes, and hamstring. Calf raises as an addition to the squats simulate the second stage which is the take off. These two exercises work hand in hand to providing a strong lift off when jumping. Landing is the final stage. Once again the squat plays a part when softy landing not to put stress on the knees or the feet.