It appears that this has been overlooked as an essential topic here on everything2.. or maybe I just can't find much about it, either way, I figured I'd throw my two cents in. I suffered through some pretty rough times that could have been somewhat prevented or at least helped if I had known some of this stuff. *grumble* At least I'm one of those people that doesn't get grouchy from PMS, I just get sad and bawl my eyes out. (I guess it can be argued which is better or worse, but at least I'm not mean to people this way.)
Here are some good ways to relieve menstrual cramping during PMS and when it finally arrives, from personal experience, as well as suggested methods I've yet to try:

1. Sex. Apparently having sex relieves cramps, but you first have to find someone willing to have intercourse with a female during that time of the month. (Try doing it in the shower, or something.)

2. Heating pads, hot towels (thanks booyaa), warm things might help to soothe a little. There is the whole heat and pressure idea, which seems to be pretty popular.

3. Tylenol or other pain relievers which generally don't do a very good job, but might help some people. Midol and other such drugs might help, too.

4. There are a ton of herbal remedies out there, if you care enough to search for them. There's an herbal ointment known as "Soothing Secrets" that is supposed to be pretty decent, and I know there are a whole slew of herbs that are nice on their own. (German Chamomile flower, Black Cohosh root, Chinese Dong Quai root, Squawvine herb, Milk Thistle seed, St. John’s Wort flowering tops, Polynesian Kava Kava root, Vitex (Chastetree) berry, Valerian root and Ginger root. These are all supposedly helpful.)

5. The fetal position (if laying down), and ibuprofin (100-200 milligrams, advil) supposedly works for the majority of people. (Thanks to Eraser_ for those helpful little suggestions.)

It's important to remember that very severe cramping and heavy bleeding can mean there is something wrong, though it really varies completely from person to person. I'm pretty sure if the cramps are really terrible it can be a sign of osteoporosis, or cancer. Either way, it pays to make sure that there isn't anything wrong. I noticed that when I was younger I had a lot more cramping than I ever do now.. ah, getting older isn't so terrible after all.

wuukiee suggests mugwort is quite helpful, and that some people find aleve to be a good pain killer.