Men have sensitive internal tissue called the prostate gland, which many men enjoy the stimulation of. It can be stimulated most directly through the anus. Even lots of straight men enjoy it. This gland produces a fluid that helps produce semen, and it closes the urine duct to allow for the safe passage of sperm. The prostate can be found a few inches inside the opening of the anus, and can be stimulated through the rectal wall. When the prostate is stimulated, some men orgasm whether or not they have an erection. 

Assuming for now that the man is on his back, slip a finger or two into the anus facing upward then curl your fingers (what some people describe as the "come hither" motion). Many men claim they like it stroked or massaged. 

When you stimulate the prostate by hand, be sure your nails are trimmed. The anus is a sensitive body cavern that can tear and scrape easily. It's not as durable for penetration as the vagina. You may want to snap a latex glove on your hand, found in many drug stores or sex shops, which will make rear entry a smoother process. There are also finger cots you can purchase that serve the same purpose. In addition, don't forget the lube. The anus doesn't naturally secrete slippery, sexual fluids conducive to easy gliding. 

Fingers aren't the only anal pleasure enhancers. The receiving male can try butt plugs, vibrators, dildos or, of course, 100% nature-made penises if that turns you on. Use your imagination on which toys you can buy, adapt, or make. If you use a toy, make sure it has a flange. A flange is a base that prevents the anus from devouring your toy, making it hard to expunge without medical services. The anus is a powerful internal mechanism that can work as a mighty suction device. 

Another thing to remember about prostrate stimulation is that not all men like it. Some find it irritating and painful. Some men really enjoy light pressure, others prefer it hard. Some like constant pressure, some like a fast "pulsing" motion. Some men love the sensation, but only like it once they are already very aroused. The best way to ensure your male sex partner enjoys himself is communication. Ask him how it feels.