The prostate is the size of a walnut and is located 4 cm in from the anus on the anterior wall, down from the bladder and in front of the rectum.
The function of the prostate is to lubricate the ejaculation process (adds to semen and other seminal fluids) and to increase pH, making the vagina more hospitable and, therefore, more conducive to fertilisation. It produces prostatic fluid when the man is aroused and contracts during ejaculation to empty.
Why do it?
Pleasure. Concentration of nerve endings makes stimulation very pleasurable for many men. Intensity of orgasm can increase two or three-fold with prostate stimulation. It can also delay ejaculation. Some men shy away from prostate stimulation because of the taboos associated with anal play in our culture. Deep-rooted (unsubstantiated) beliefs leave men feeling shame and a general concern that they will become or will be considered homosexual.
Talk about it
Talk to your partner about whether he is open to exploring the prostate. While giving a blow-job you can show him you're comfortable by playing gently with the outside of his anus and perineum (area of skin between the anus and the testicles).
How to do it
Position: The most effective way to access the prostate is anally. Have him lie on his back with knees up; this leaves the penis available to play with throughout the prostate play. Sit between his legs.
Lube up: Thicker lube is better for anal play.
Fingernails: Keep fingernails short. Latex gloves prevent bacteria lying under the fingernails to get into the delicate rectal tissues. If you have long fingernails, put cotton balls over the nail, inside a latex glove.
Finger position: With the fingernail pointing back, bend your finger up into a "come here" position. Insert a finger when he is aroused. Slowly rub the outside of the anus, smoothing lubricant fully around the area. Slowly slip in the tip of the finger; this will cause the two sphincter muscles to contract. Give him a minute to adjust to the feeling. Watch his facial expressions and body movements to guide you. All the while continue to rub his penis. Move the finger in and out gently at first, more vigorously, depending on how he is responding.
The urge: He may have the urge to urinate or have a bowel movement, but he won't. This is just apart of the process of stimulation.
External access: If he is too worried about anal play, there is always another (albeit less effective) way to access the prostate. The prostate can also be stimulated from outside the anal region, although not as effectively as internally. Apply pressure to the area between the scrotum and anus. Massage.
Right tool for the job: Start with something small and work your way up. Be sure to use toys that have a flared base to avoid the anus' natural suction. Check out these toys especially designed for anal play.
Adapted from www.libida.com