from what I've observed, experienced, and read, the most important factor in producing powerful and voluminous ejaculations is to have a strong orgasm.
To have a strong orgasm, you need to build up neural energy, especially in the erogenous tissue involved in producing that orgasm.
Edging is a very successful technique to do this - do a google search on edging and orgasm and you'll find information.
The other less talked about factor is circumcision. Circumcision often removes vast quantities of erogenous tissue, in particular the frenular area. This is why the strongest ejaculations are by and large produced by intact males.
Pay attention when you watch porn, and you'll notice that the men who shoot really hard - those powerful spurts which fly with so much force that the sperm splashes on the target 9+ times - are almost all intact. Not all intact penises look like manuel ferrera's - many of them look like circumcised ones when they're erect. They thing to look for is the frenular area on the ventral side of the penis - the frenulum is a big give away.
See my thread on circumcision and ejaculation here:
http://www.lpsg.org/89551-disturbing-observation-intact-vs-circumcised.html
That said, there are many things you can do to maximize your own potential, intact or not.
Taking the time to build up arousal, edging, good nutrition and mood will probably all contribute to more powerful orgasms.
Most importantly, experiment by using different types of stimulation. With patience and trial and error, you will discover the techniques that work best for you, and will probably discover new secrets about your body and sexuality well into your old age.