Stimulation of the prostate through mechanical/sexual methods is the basis for what people claim is the male version of the infamous G-Spot. Stimulation and triggering of the prostate is hardly the same as the female version of the "spot," but with the male we at least have an anatomical feature that can be associated with such a sexual reflex.
The prostate helps create seminal fluid. By manually stimulating this gland, either through anal sex or through digital palpation (a finger or artificial penis), it tends to set off the prostate and men ejaculate. The response and feeling is different from men to men. Some men feel glorious orgasmic pleasure when it is stimulated, some men don't feel much apart from spinal reflexion and the ejaculation response.
If a man wants to stimulate his prostatic g-spot, or if his partner wishes to do so, one simply needs to gently palpate (feel) for this small gland. It's about the size of a fairly-nice-sized walnut and is accessed through the external rectum. It is located anterior to the rectum. Meaning, it is between the rectum and the penis, not between the rectum and the outer edge of the gluteal muscles (when visualizing it from a lateral viewpoint).
Palpation should be done very cautiously and carefully. Using a gloved and lubricated finger is best. Anything introduced into the rectum should be sterile, obviously. Having the man "bear down" will help with the introduction of any object into the rectum. About one inch into the rectum, simply curl your finger toward his penis, and you should feel this gland. Be gentle with it.
All men are different, of course, but some men will orgasm immediately upon palpating this gland. Other men may take a bit of persuading and patience. Ejaculation is usually a sequel of this palpatory event.
There is also another area of/on men (and women, too) that has extreme sexual-sensory recognition to stimulation. This is called the perineum. It is located (externally) in the mid-line between the vagina and anus in women. In men, it is between the testicles and anus. Lightly rubbing or applying degrees of pressure (it varies by person and their tolerance) with a finger or tongue to this area is certainly useful in lighting your partner's Christmas lights up (and pretty brightly, too, I might add!). Stimulation of this area can trigger an orgasm that can be extremely intense. In some cases, it is even more intense than a conventional orgasm. However, these orgasmic responses hardly ever result in a release of fluids (when orgasm is triggered in this manner). This is for both men and women.
About me: Exploring unconventional methods of sexuality; methods that our ancestors enjoyed and impassioned themselves with, is one of my interests. Those men and women who are interested in autofellatio should read Yogafellatio. It is a book that helps guide men down this sexually exciting and interesting path.