This is best done with a partner. My wife and I recently demonstrated our technique for two of her friends who want to try it on their men.
I will settle back on a nice sofa or lounge and my wife will sit between my legs and masturbate me for about 30 minutes or so. When I am close to coming she will lift my legs up over my head and slowly insert a finger into my anus. When she reaches my prostate she will flick it two or three times which instantaneously causes an outrageously large and pleasurable ejaculation - ususually accompanied by a howlingly good scream. The ladies were impressed.
Get a partner!