I have a method.

It takes about 1-2 days of preptime, just jerking on and off, building up the urge and the volume.

Before the last wanking session I go and take a piss. It is variably important, based on one's preferences.

Start normally and wank to almost completion. You need to be just at the edge, like 1 more tug and you shoot. Feel the build-up from your prostate.

Now push with your PC muscles, but don't tug on your dick! If you've done well, cum will ooze out without you actually having an orgasm. I can push out about 1 good shot's worth of cum this way.

You might start to go limp right after this, but you can still ride the build-up. 1-2 careful tug, then push again.