WARNING: LONG POST
I'm sorry if this ends up being a long post; I am the broken joint/muscle issues gal thanks to a life of chronic pain due to creative (genetic) joint/muscle/ligament situations (read, "born just not right")...
Question: When did it start, and did it coincide with an activity (like knitting...) that may be causeing muscle/joint stresses? If yes, then it may be time to sort out some ergonomic (happy for your body; I've had to become the ergonomics queen to get through the days) to remedy the situation. More to follow...
Your joints/muscles work in systems; this we all know. For our hands (and feet, though it's leg not arm related) the systems that affect us are mostly found in the lower arm/upper arm/shoulder areas. The muscles and tendons that make your fingers move are mostly (but not entirely) located in your arms. What this means for you is that the root of your problem may lie in your arm, not your hand.
So, for self treatment, try these simple loosening techniques:
-take off your jewlery, watch, etc to remove any small (but related) weight from your hands and to clear your field.
-gently shake your hands in front of you, over your head, to the side of you, then hanging (with arms limp and fully extended) at your sides. This should (without your even trying) engage the muscles of your hands, arms, shoulders, and neck and help loosen them.
-while cradleing one hand in the other, rub the palm of the relaxing hand with the thumb of the cradling one around the meaty parts of the outside of the palm, under the first knuckle, around the fleshy part of the thumb, etc around the whole outer edge... do both hands.
-using the index and thumb of the massaging hand, rub the fleshy part between your index and thumb of the relaxing hand with as much pressure as seems right; you should feel that pleasure/pain thing one gets when rubbing out knotted muscles... do both hands.
-rub up the wrist, from just blow the hand/arm juncture with the thumb and fingers of the massaging hand. For me, this involves holding my massaged hand in front of me, inside of wrist facing my body. The rubbing hand goes thumb on top (on the inside of the wrist), fingers on the back. Rub in small circles, paying special attention to that hand/arm juncture - there are lots of small bones in there that could use a rub down. Work your fingers up your arm to your elbow slowly, working both front and back. Do both arms.
-turn the massaged arm over and massage the muscles/tendons that run along the top of the lower arm with the finger tips of the rubbing hand. These muscles get REALLY jacked up on my body, and cause all sorts of weird problems even when I don't feel pain... Do both arms.
-rub the bicep/tricep with a pincer like gesture of the rubbing hand (thumb being one arm of the pincer, fingers the other). Get the front, back, and sides... Do both arms.
-rub the point of the shoulder (on the arm side, not the shoulder side), paying special attention to all the bits that feel sort of stringy. Work across the strings as well as up them. Do both arms.
-give yourself a quick should massage by relaxing the rubbed arm and crossing your rubbing arm over your front to rub the muscles in your shoulder, shoulder/neck junction/neck area. Do both arms.
-shake everything out again.
I personally like to do one arm from hand to shoulder at a time, then switch to the other arm (as opposed to each section of the arm on each arm at a time).
This all feels much better when you have someone else do it. I reccomend an exotic, hot person of the desired sex, but a loved one or friend will also do nicely (or fit the exotic and/or hot bill); you can discuss knitting/the weather/good books while doing this. Avoid politics, troop movements, foreign policy, etc as they can lead to muscle tension.
One massage techniqe I was recently showed that is AMAZING for both tension release and a headache cure is to have someone rub the fleshy part between fingers and thumb, then to find a spot there and squeeze until it hurts (you tell them to stop squeezing at that time). They hold that pressure until the pain stops (you tell them when it stops), then sqeeze harder until you tell them to stop. Rinse, lather, repeat as desired. Move from spot to spot until they are sick of squeeing your hand. Works well while watching TV.
Heat therapy is always good for muscle weirdness. Remeber to treat the system, not just the offender; if you are having problems in your hand, remember to get your shoulder as well. A nice hot bath/sauna/hot-tub can do a lot to relax the whole body... baths are easiest and can be nicely accented by lavender oil in the water (relaxing), candles, mellow music (no speed metal, unless you could fall asleep to it), and a do not disturb sign (unplugging the phone might be good too).
As tammynelson said, various vitamine deficiencies can cause twitches/cramps as well... bananas area good source of potasium (a deficienciy of which can cause cramping and other weird stuff). Multi-vitamins can also be helpful for filling in vitamin blanks. If you're a girl, womens specific vitamins often have more apropriate percentages of the things girls need - like potassium and calcium - than gender-generic vitamins.
Drink lots of water; many muscle issues arise from slight dehydration. Grab yourself a decent sized bottle of water (I like and carry litre sized to work and school and drink all the time), disregard the "do not refill" warning on the bottle, and drink drink drink water. When you get up from bed, while you knit, while you work, before sleep, while driving (being careful to watch that pesky road), on the phone, etc. Drinking lots of water can also help avoid illness, cure hang-overs, and stop pesky dry mouth problems. It is a good thing to pee more than normal because you drink more water.
As for herbal treatments, chamomille is good for many (though I'm allergic and have all sorts of problems with it, so if the tea form smells bad to you, I reccomend not using it in any form). Vallarian root is also good; a little web-research and following the directions on the bottle is good for dosing. Vallerian certianly can cause drowsiness (it can be good for helping you sleep), so being home and relaxed is reccomended. There are certianly other herbs, but those are easy to find and effective for many people.
For knitting specific issues, earlier threads have discussed the differences between metalic and wooden needles; some people find that wooden needles, while expensive, can do much to relieve pain/tension for some reason. Me, I don't find they are helpful, but my body doesn't respond like many people's does, so experiment and see if wooden/bamboo needles help. Try taking brief breaks while you knit to shake and rub out your hands.
If these, or a combination of these, don't help, then as reccomended, massage/chiro may help. Accupuncture can help with lots of things, not just hand muslce twitching.
Good luck, fun rubbing, and knit on. If something here was unclear, or totally boring and you want to tell me to shut the hell up, please let me know