squeeze your dick tight when the first "throb" comes that would shoot, as to not let the cum out yet. Then when the second "throb" comes, let er fly!
I've found that a period of remaining celibate may increase the density of the semen & ease of reaching a climax, but not the power of the ejaculation. On the contrary, I find that forcing numerous orgasms, while the semen may become increasingly watery on each successive occasion, the force behind it is increased.