| Contrary to popular belief, it is possible for men to have multiple orgasms. The easiest method for males without dedicating years to studying the Kama Sutra and tantric sexuality, is to strengthen the pubococcygeus (PC) muscle. The PC is the muscle responsible for contracting the prostate forcing semen to be ejaculated. This is a very weak muscle as it is not used but a few contractions each time a man ejaculates. By strengthening this muscle, a man has the ability to control it during orgasm thus preventing ejaculation from occurring. Since ejaculation is what signals the body that sexual intercourse is complete, preventing ejaculation allows the penis to remain erect after an orgasm, thus allowing continued intercourse.
Strengthening the PC muscle is not an easy task. The best way is, of course, to exercise it. First you must figure out how to control this muscle. When you urinate, typically at the end you may contract a muscle "down there" a few times to expel the last few drops. That is the muscle, and that contracting you do to shake out the last of the urine is the same contraction you want to do. Another way to think of it, when you have an erection and you contract your muscles to make it move up and down, that is the muscle and the exercise you want to do. Several times daily contract and release that muscle in sets, as many times as you comfortably can. At first you may not be able to do more than 10-15 at a time, but with time you should be able to do several hundred a day.
After doing this for awhile, at least several weeks (note: ok, 4-6 weeks may have been a bit much, but I kept forgetting to do my exercises...), you should notice that your orgasms are improved. At this point you're getting strength built up which will allow you to control your orgasms. To do this, as you feel your orgasm coming, you tightly clench your pc muscle to prevent it from uncontrollably contracting. For a while, this will require intense concentration and a lot of effort to hold the muscle. But, you should get some partial success here. You may have times when you ejaculate but don't have an orgasm (that sucks big time), but generally your first sign of success will be a partial ejaculation, where you do ejaculate some but you don't really contract your PC muscle, so that a few seconds after this you can quickly have a powerful orgasm. Enjoy your first sweet taste of victory! Given time, it will become easy. Once you can control this and prevent ejaculation, you will then be able to achieve multiple orgasms without ejaculation.
While this may seem like a lot of effort, and it is, it is well worth it. As many women can attest to, each subsequent orgasm in a series is more enjoyable. If you enjoy the feeling of an orgasm (and who doesn't), you will not believe how much better the third or fourth one in a series can be. You will also have the added advantage of having complete control over your love making, with the ability to go on as long as it takes to please your partner and experiencing several intense orgasms along the way.
donfreenut's advice is good too, but it can be kinda painful at first :) Sacrifice the body for the cause my friends! | 
