I've actually found, in the last couple of years, that delaying, holding it as long as possible, right at the moment of organism, really squeeze your muscles like you're trying not to pee, and just try to hold it a little longer every time. It's one of those sensations that kind of drives you a little crazy at first, but also feels crazy good. You just hold that moment of pressure as long as you can and then push out as you let go. I've developed this technique just from watching lots of porn with big cum shots. This is how porn stars get that affect when they do facials that makes it shoot out with great force all over her face. I cum harder since I gradually started doing this, and I'd say I cum more, volume, on a regular basis. Obviously what everyone has said about abstaining for awhile is the best way to make sure that next shot is huge, but other then that, the technique I describe is the best way I've found at improving my cumming skills.

Also, if you're holding it and you start to leak a little cum, keep holding it as long as you still feel that pressure. You'll sometimes feel a little like you've cum almost your full amount before you let go, but whatever is left will usually still pop out with great force and sometimes distance and still feel good. Plus, that is just a cool looking thing to watch or film your cock doing. It looks great, all red and swollen as big as it gets, and you gradually start seeing cum ozzing out the top and then finally shooting straight up. Anyway, just my experience.

Videos of this are my favorite. I love it when I just "ease" over the top, then explode.