The technique of prostate massage actually involves two (three) organs of the male reproductive system: the prostate gland, the seminal vessicles, and the ampulla, which is the reservoir which actually holds the male sperm just prior to ejaculation. The prostate gland when engorged can be felt by pressing a finger into the middle of the perineum (in back of the scrotum). It is a rounded lump about the size of a large marble. It can also be accessed via the rectum, where it can again be felt as a rounded lump toward the front of the body and just about an index finger's length inside the rectum. A better way to find it from the rectum is to insert the middle finger, and slide it from just inside the rectum (on the front side) up and in until you feel it move over a lump. That will be the prostate. Just in back of (up from) the prostate is a smaller and roughly triangular wedge shaped nodule that is the bottom portion of the somewhat larger seminal vessicles, which is where most of the fluid of the ejaculate is produced and accumulated. Underneath the seminal vessicles are the ampulla.
During sexual arousal, sperm and ejaculatory fluid accumulate in these glands backing up behind valves in the ejaculatory ducts. When fluid pressure reaches a high enough threshold, the valves open and the urethral bulb fills, triggering the ejaculatory reflex and muscular contractions of orgasm, which empties the glands.
The seminal vessicles, ampulla and prostate can also be emptied by massage of these glands through the rectum. Rhythmically massaging the organs in an in-and-out motion with an anal dildo is one way, but perhaps a more reliable way is to simply use either the middle finger or the middle finger and index finger together (well lubricated), to apply a firm slow massage over the nodules of the glands inside the rectum. The massage will force the seminal fluid and sperm past the valves in the ejaculatory ducts, and empty the prostate (which discharges directly into the urethra). If arousal is low or penile stimulation is stopped for a brief time before the prostate massage begins, draining of the ejaculate into the urethra will not trigger orgasm. Once ejaculate appears at the meatus, it will help to empty the urethra by running a finger firmly up the underside of the penis from the root to the head, pushing the ejaculate in front of it.
A variant of the prostate massage that is quite effective in limiting any tendency to orgasm and the pleasurable sensations of fluid passing through the ejaculatory ducts and urethra, is the "ice massage": at least 10 minutes prior to the prostate massage, plastic bags full of crushed ice are packed around the male's cock and balls. The bags remain in place during the prostate milking. When the massage is completed, the bags are removed. This technique also makes a very appropriate opportunity to install a chastity device on the male's shriveled penis. (This technique works best if the male if fastened down - it's very hard to get a man to stay still when his genitals are being frozen.) Naturally, hubby will not be at all pleased to be denied the small amount of pleasure of the surrogate orgasm as the milked fluid is forced through his numbed ejaculatory ducts.
A condom may be used to collect the ejaculate, so that the mistress can gage when an amount of fluid normal for an orgasm has been collected. At this point the prostate massage can be ended.
While the prostate massage is very pleasurable, most men are unable to orgasm by prostate massage without some kind of penile stimulation. Once a sufficient amount of ejaculatory fluid has been drained, insufficient fluid pressure to trigger the ejaculatory reflex will remain, and no amount of penile stimulation will result in orgasm until the fluid glands are recharged. This may take several hours to several days.
This technique may be used to frustrate male masturbation and orgasm if applied at appropriately timed intervals. That may need to be once every day or two unless the male is secured in a chastity belt. Belted males will still need to have their glands drained every few weeks to maybe 3 months in order to avoid nocturnal ejaculations with orgasm. With regular prostate milkings, a male may be deprived of orgasmic release for an arbitrarily long time with no deleterious effects.
How long to go between milkings will have to be gauged by experiment. You want to go as long as possible without triggering an orgasmic wet dream, since the pressure of fluid buildup increases the urgency of orgasm denial. That's the whole point, right? The longest I've gone without an orgasm in my adult life is maybe a month or so. While I didn't have a wet dream, I had rock hard and throbbing erections within my chastity device after about 10 days or so. I'd guess that 6 weeks between milkings might be a good start. Then add a couple of weeks to that each time between milkings, until a wet dream is triggered (but don't go longer than 3 months between milkings). You should add a clause to your contract that hubby must confess any wet dream or other spontaneous orgasm. I'd suggest that you restart the one year chastity period from that day if one occurs. He's already had his orgasm, and will have to wait the full period till the next one.
A particularly effective way of increasing an orgasm deprived male's sense of sexual frustration between prostate milkings is to masturbate him once a day every day to just short of orgasm, then install a chastity device. I'd say this at least triples the urgency of orgasm denial. If you do this, you will need to milk him more often, maybe every 2-4 weeks. After milking, the cycle can be started over. One woman I know sometimes masturbates her hubby to just short of orgasm multiple times a day between milkings!