I noticed that sometimes when I play guitar, I have some pain in my left wrist, the one not picking, in my case. It almost never happens, but when it does, I need to stop playing for a few seconds. Basically, it's nothing, really. But still, what should I do to prevent it from getting worse?
Is it bent at an sharpish angle? It doesn't even have to be all that sharp to cause problems. Hold your arm out and relax, you'll notice your wrist is curved but not really bent all that hard. That's the natural position for it, and if you bend it too far beyond that and do something like play guitar you can do damage.

It could also be caused if you have to much tension in your hands, i.e. your gripping that neck too hard.
Maintain as neutral a wrist posture as you can. Bending your fingers while your wrist is bent causes the tendons to rub hard on the little hoop of bone/cartilage in the wrist that acts like a pivot point.

Alternatively, go hit the gym.

Or, drop a heavy weight on your foot so that you can't even remember that your wrist hurts.

After I broke my neck in a car accident, my wrist and finger pain didn't seem as significant anymore.
All of the posts relating to relaxation and natural position of the hand are definitely something you should explore. Remember, the goal is not total relaxation (otherwise playing would be possible), but it is learning to be aware of what are necessary or unnecessary tensions, and how to find play with the minimum tension and effort possible for a situation.

Guitar playing is a relatively unnatural activity. Your forearm, wrist and hand is rotated through 90 degrees from its natural hanging position at your side, and there are times when bending the wrist is necessary. The force of the fingers fretting notes is towards you, another thing that makes guitar playing counter-intuitive and physically unnatural.

The first thing is to find a strap-length and guitar position that is not either so high or low that it forces you to lock your wrist in a drastically bent position. Once you've found a playing position where your arm feels comfortable as a unit, then you should try to "relearn" playing, constantly trying to reduce tension whilst maintaining results and increase efficiency. Gradually, this becomes something that comes naturally, and just requires the thought of reducing tension. This requires a lot of concentration, as you have to isolate movements whilst not allowing anything to tense/move. Just take it very slowly, start with simple exercises and then gradually build in complexity, and make your primary consideration the elimination of unnecessary tension (you shouldn't just be concentrating on your hand/arm, that goes for your whole body, too).

Warming up properly, so that there's a lot of blood in your hands/fingers before you do awkward stretches or contractions is really important, too. Ideally, you should warm up for about 5 minutes before you touch the guitar, then spend about 5 minutes on finger agility exercises with the guitar in order to get the hands warmed up, and then continue with any other technical work that you like to include in your warmup. Finger agility exercises (particularly trills) are a great way to get the hands warmed up. Practising in really long sessions isn't great for your muscles, and taking regular breaks to stretch, shake out your muscles is really important.

Apart from that, massage is a powerful tool, and any guitarist who plays for a substantial amount of time each day, or anyone using their hands in a similar way for hours a day should be aware of some really simple exercises you can and should do anytime, whether you're waiting for a bus or sitting on the couch. You also shouldn't wait to get pain to do these, athletes take far greater care over their bodies than musicians do with their hands.

1) Massage in between your thumb and fore-finger. This often hurts, if you've never done it before and you've been playing hard. The pain is an indication that you should continue massaging, but be gentle, don't press too hard, and over a period of time, your hands will soften and pains will be greatly diminished.
2) Turn your wrist so your palm is facing upwards. Feel in your wrist for a number of thin bones. Gently manipulate these and move them around as much as possible. This will improve flexibility in your fingers, and this is really good to do before you start playing. Try doing an awkward stretch or chord on your guitar before doing this exercise, and then try the same thing afterwards. I've seen people do this and be amazed and how easily they get what they found so difficult before, and even be able to stretch further.
3) Hold your arm out in front of you, palm facing down. Find where your forearm muscle ends at your elbow, and massage here. If you're getting pains in your hand, this is very likely the source: the fingers don't have any active muscles of their own and are controlled by the muscles in the hand, and the muscles in the hand in turn stem from the forearm. This forms a whole chain, your forearm is obviously linked to the upper arm, which goes through the shoulders and down into your lower back. Being aware of this relationship helps you become aware of tension in your playing, and also how to treat it yourself if you do get pains.
4) With your arms completely relaxed at your sides, feel your hands hanging as dead weights from your shoulders. Feel your armweight going into your finger tips. Gently rotate your hand from side to side in very small movements. This shaking of the wrists can really help you to relax your arm, and can increase the effective length of your arms by up to an inch.
Last edited by National_Anthem at Jan 10, 2012,