Do you mean standing vertical leap (no running start) ? This ability has a very strong genetic component, the muscle fiber composition of your lower body has to be primarliy fast twitch fiber. How high can you jump now, and just as important how much can you squat ? I'm not certain but I do feel there is a correlation (for me personally at least) between your sqauting strength and your leaping ability. Also are you a fast natural sprinter, I feel this too is a big factor of how good of a vertical can be acheived.
In any case, a 8 inch increse in vertical is probably impossible. You should be able to put 3 or maybe even 4 inches on your vertical through lifting (esp lower body read:SQUATS), sprints, and keeping bodyfat under 10%. But at the end of the day I beleive your vertical is limited strongly by your genes. When I first had my leaps tested at 16, i didn't even work my legs and jumped 38". After four years of heavy lower body work (squat and deadlift just shy of 500 now) I jumped a 41" . I'm only 5'8' so its kinda useless but still it is cool to freak people out by being able to almost fly.
What I would do is hit the legs hard once a week. By hitting legs I don't mean leg-press, leg-extension, leg-curl, and hack squat. These movements are completely useless in my opinion. If you want strong legs, you squat with big weight, period. Yeah I know there are dudes out there who can jump 40" plus and don't work legs at all, but your probably not one of them so you MUST squat. Also practice the movement. Just like you would lfit weights, do 10 singles like grabing a basketball rim before or after your sprinting sessions. Sprinting is also great for your hamstrings, and I feel a faster sprinter is a higher leaper too. So hit your legs hard in the gyn once per weak, and do a sprint/leaping workout twice per week and you should increase your veritcal leap, but most likely not by 8".