Who it's for, and when
Who it's for: men curious about new sensations, alone or with a partner, and partners who'd like to give them this pleasure. Prostate pleasure is nothing like the pleasure of the head of the penis, emotionally or physically. A bit like the difference, for a woman, between a caress of the clitoris and stimulation of the G-spot.
No hang-ups: this caress is still taboo for some men, and the idea may unsettle you both a little. Nothing abnormal about that, and nothing new either: in the old days, they called it "postillionage". Take it one small step at a time: you might just discover something unexpected.
When: never straight in, always after plenty of caresses, when he's already very aroused. Pick a moment when you have time to spare, with no eye on the clock.
Precautions: a strange, even unpleasant sensation at first is part of the discovery. On the other hand, pain that persists, trouble urinating (the prostate changes size with age), an infection or a known prostate problem call for a doctor's advice first. A flare-up of haemorrhoids or an anal fissure? Wait until everything has settled down. At the slightest bleeding, stop, and see a doctor if it happens again. And if you're not supple enough to massage yourself, your partner can reach the area from an easier angle.