You get things like RSI CTS Tendonitis etc (all the names mean nothing, it all comes down to the same things) through bad posture (not just your sitting/back, this means position of your hands, fingers, feet, neck, shoulders) and tension.
Playing the guitar requires training of lots of different muscles, these muscles are joined up. When you use certain muscles, others try to help, because you haven't yet learnt to isolate the muscles you NEED to use. So when you try and do things with your fingers, your arm muscles tense up, trying (badly) to help.
Ever notice when playing something really hard, you can relax your leg muscles afterwards because they were all tensed up, or perhaps the most common one- THE FACE!.
If you allow these tensions to continue (they will be apparent when you are trying something new) your muscles become worn and damaged.
You have to try VERY hard to relax these muscles, the best way to do this is to play VERY slowly (ever tried playing at 1 note per 4 clicks at 60bpm?) and making sure you have no tensions, doing this a lot, slowly building up speed.
This is not a speed building excercise (although it helps) what you are doing is making sure you are playing with no tension, then speeding yourself up to the point where you find your self tensing, relaxing that tension, and then carrying on.
If you try and play something now, that is way beyond you (say semiquaver-sextuplets at 200 bpm), you will find you tense up to try and do this. Now if you watch some of the big players playing fast you will see they are completely relaxed.
I had a lot of problems like this, and my main points of tension were the tricep, the forearm, and the shoulder and my hands. I found myself pulling my elbow in towards my body when playing difficult passages. When you are playing, someone should be able to grab your elbow and swing it around. Find your own tensions and deal with them. This is something Yngwie mentioned when asked about his meeting with RSI, he said 'you have to find your problems and fix them'. This is something YOU have to do, you have to discover it for yourself.
Stretching well before playing is also a good idea, google for basic yoga positions or sports warmups.
If you want some great examples of relaxed fast playing check out videos of
Shawn Lane (so far the player with the best form/posture/technique I have seen, is it also a co-incidence that was one of the fastest players?).
Guthrie Govan (although he displays facial tensions, this is more related to the music he's playing, not physical tension, he plays many VERY difficult licks with a motionless face)
Yngwie Malmsteen (ignoring all the kicking/pointing/guitar throwing, this guy barely moves when playing, he really isolates his hands.
Michael Angelo Batio (really efficient playing, really bad music)
Steve Vai (brings me to my next point)
You may say, but guys like Vai are always making faces, raising their shoulders and looking like they are about to explode, whats wrong with me doing that?
These guys are IN CONTROL of these movements, they are not being forced by their bodies to make them. They are either consciously making these movements (stage presence etc), or they are in reaction to the feeling from the music. Now do NOT try and use this as an excuse for tension, any crap like 'but what i'm playing moves me so much that I have to tense up' is rubbish. If you are tensing and playing excersizes and sequences etc, I doubt this is the 'music' moving you. I absolutely guarantee that when these big players practiced they were completely relaxed, they were practicing the movements, not performing and playing their music.
wow, rant.
