practice when masturbating.
With a bit of practice and proper breathing and relaxation, you can masturbate for as long as you want without building up climax. Then you can "let go" and build up orgasm and cum.
It might help if you think of it as a passive thing. Each stroke/whatever is like the first, or only stroke. Breath with every stroke, you should be able to "recycle" your sexual energy with your breath and make for a continuous, replenishing cycle.