PMS may be a joke to those who have never felt it (i.e. men), but it is a really huge problem to many women. It has been turned into a joke by many, disregarding any irritable behavior of a woman as "having PMS" without knowing what it actually is and what consequences it may bring.
Most women (anywhere from 80 to 95%) have felt this annoying and even detrimental collection of physical (such as swelling of the abdominal area and breast tenderness), psychological (depression) and emotional symptoms, all of which are nothing short of bad news for the male element of the house.
The symptoms of PMS vary from woman to woman and month to month but the most common ones tend to be present every time: most women suffer from depression, a term used freely and not necessarily meaning a physical condition, but more likely a lack of desire to do anything, thus leading to a milder or more extreme form of apathy, sadness and so forth. Some of them feel so "depressed" that they can't go anywhere and stay at home vegetating and not being able to stomach the pleasant sound of soccer games on TV.
Another popular symptom is sensitivity and irritability, typically the one most noticeable on the outside. If this is noticed you should be gentle to the woman and try not to vex her in any way or you may fall victim to the seemingly endless flood of crying, cursing and hysteria, which is undoubtedly an uncomfortable feeling. Compliments are the most simple, yet effective, remedy to this symptom, so it is advised to premeditate those in great advance. Do not overdo it, though, because, due to the ever-active irritability and sensitivity, a compliment, that a woman during her PMS deems an obvious lie, will surely offend her, hence the extra crying, cursing and hysteria.
Tender though men usually tend to be concerning women (and with good reason, one might add), they may have to turn the dial to "excessively tender, moreso than pink furry bunnies may seem on the best of their days", which is obviously a difficult, if not impossible thing to do for the average beer-chugging specimen of the primal kind.
Probably the most dangerous of symptoms due to the delicate nature of females is the feeling of being "not pretty", which has a lot of other more specific terms, such as "too fat", "too skinny", "too wrinkled" and the less common, albeit decisive, "I have an appointment with my plastic surgeon". The task is then more difficult than it may seem, for saying "you're not fat" is never enough (and it is hard to even imagine the consequences of "plastic surgery will fix nothing"). This brings about many a problem ranging from the most obvious monetary ones to the even more annoying constant stalling in front of any and all mirrors in the house, most notably when it's time to leave the house.
The best idea is to use actions instead of words. A dinner in a posh restaurant, a present, a kiss and a hug, along with the ultimate one explained in the paragraph below, is the best way to tame the beast that PMS is.
The worst part, however, are the unexpected mood swings. In the best traditions of Stanley Kubric's "The Shining", a woman may be nice, loving and caring one moment, and searching for "that axe that you used to chop down the god-damn Christmas tree with" the other moment. These back-flip changes are seemingly unpredictable, but one can save himself by handing her an insane amount of sweets (preferably - chocolate) and a TV program with old melodramatic movies the times of which you have, preferably, underlined in advance. This method is not good for her (and may worsen the "I'm not pretty" symptom), but when lives are at stake, it is perfectly legitimate.
Chocolate and oldschool movies are actually a means of salvation in many cases, so a man should have stashes available at all times. The geographical placement of these stashes is down to strategic planning, so experience from such things as computer gaming (and tactical allocation of certain adult entertainment magazines) is very welcome. The list of these places would typically include the bedroom (under the bed or near it), because of the insomnia which drags together with PMS. Also, the bathroom - a subtle destination which some find effective and others don't. The bathroom stash is a way to counterattack the "I'm not pretty and therefore shall stand before the mirror in vain for hours and not let you shave" situations. And the crucial living room/kitchen stashes.
These guidelines should get you through a woman's PMS with only slight bruises or the occasional dislocated joint, so it is advised to follow them with immense vigor.