[quote name='GuilewasNK']I'm 6'6" and 32. I haven't played since my college days but I was able to dunk in the 8th grade. I can still dunk, but not like I used to.
The best exercises are seated leg raises and high-knee running. Those will help with explosiveness. Stay away from squats, it's too easy to injure your knee. I injured mine in high school doing squats during off-season. If you decide to do leg presses on a nautilus machine keep the weight reasonable. You don't want to get your legs bulky. keeping them flexible is the way to go. Get too bulky and you can risk hamstring injuries among other things.
A drill that can help jumping endurance is the Mikan drill. It's a lay-up drill, but it can really help your jumping endurance.
http://www.youtube.com/watch?v=jYUVMGDL1rI[/QUOTE]
I think Guile is spot on. Bulking up through squats is not the key and will likely actually hinder you. If you are going to do any bulking up, I would do calf raises (seated or standing). When developing the leg muscles, I would also recommend glucosamine for your joints. Other than that, nothing is going to get you there except actually doing it. So you have to work your way up to jumping high by jumping as high as you can, then a little more and so on. To do this, like Guile suggested, Mikan is one great workout that will train your muscles. Another one is box jumping to gradually higher levels.
To summarize, I think the key is Mikan drills and plyometrics/box jumping. If you need additional help, you might want to look at calf raises and glucosamine supplements (helps repair and strengthen joint cartilage).