Ways Of Preventing RSI
01. Check Your Fretting HandHaving a poor hand position can be a factor is having more tension in your fretting hand, this is usually because the placement of the thumb is too high and that the wrist has to bend to allow the fingers to do their job. Depending on the size of your hand and how comfortable you are with playing with your thumb quite high this should be seen as a problem that needs sorted -- look at your wrist and that should tell you everything.
2. Check Your Guitar HeightNever ever play with a guitar too low down. It does not look cool, it does not enhance playing but it does encourage injury. Playing with the guitar quite high is actually more comfortable and allows for better playing in most cases. Again the indicators here should be the wrists and how straight they are.
3. Check Your Picking HandAnchoring your hand, forearm, pinky or any part of your body to the guitar creates more friction and more tension in your muscles. Fact. The decision as to whether they create enough tension to be a problem is yours. If it is then it would involve learning a new way of picking all over again -- this is worth it as I'm sure you'd rather play guitar than not even be able to grip things (happens in extreme cases).
4. RelaxLike above -- any unneeded tension can attribute to more pressure on your limbs. This can result in having a sore shoulder when you play to your hand going numb. At no point is guitar supposed to hurt when you're playing (apart from calluse building in your finger tips). If you are having such problems then just try to play something easy with no tension -- think light. Sometimes tension is a result of muscle memory and needs to be wiped out through lots of practice. This should involve relearning something slow -- speed when you can't play very fast causes most people to lock up their hands a bit too much.
Guitar ExercisesThe usual exercise of going 1-2-3-4 on the E string and moving up a string and up a string is good as it forces you to use all of your fingers and is a good indicator of improper technique. Your fingers should be able to easily fret after a little bit of practice and some improved finger independence. If this isn't the case then your thumb is probably too high.
Warm Up ExercisesAthletes warm up before running -- guitar is a physical exercise too. Try creating different stretches and such to help counter RSI. Something like putting your hand out before you like you were high-fiving someone, straightening your arm and gently pulling back on all of your fingers with the other hand until you feel a stretch in your wrist is a good little warm up.
Also try to stretch the wrists gently in any other direction you can. You might also considering doing stretches for your picking arm too if you are just geeting use to not anchoring as this arm is going to be feeling tired at the end of a playing session (tired not sore -- it's muslces that need to build up).
Build Up EnduranceThe strength of the wrist depends on how strong your forearm muscles are. So some good exercises would be something chin-ups if you are lucky enough to have the facilities and the strength. Otherwise there are other exercises you can use.
For example, I use a tennis ball in each hand when I'm walking somewhere or waiting on a bus or watching TV and, with all of my fingers over the top I sit and squeeze them. This would build up muscles in your forearms and would also improve finger strength too. The stronger a muscle is the more it can endure before injury.
Golfers are known to always carry strong rubber balls for the same purpose and many of them (due to increased muscle in their wrists and forearms) have longer drives than before. Padraig Haddington uses a product called Powerball that does the same job except it puts a small amount of pressure on your muscles indirectly. It is recommended by chiropractors which brings me to my last point.