Once discomfort kicks in:
* Do exercises indirectly affecting the pained muscles. Dancing, walking and most cardio exercizes evelate strain in back and neck muscles. Avoid exercises doing actual movement with the muscles and try not to massage or stretch the pain the away. Because the pain is a signal of the problem but fighting the pain not alway removes the problem and good indication of how the problem is progressing.
* Switch hands, its hard to become double dexterous pc user but it costed me just 2 days. So I wouldn't call it a big challenge either. Surely there is function loss using the less experienced hand but switching it off removes the strain time.
* Stress, 15 minutes of stress working in ms word puts more strain on your muscles than enjoying a casual game all day. Even if the gamer guy was slouching up straight the test person using ms word always showed more strained muscles and discomfort after those 15 minutes. One could think that if you were playing a game that added similar stress an hour could still cause rsi.
Once it's gone:
powertraining or swimming, basicly shit to increase general muscle strength and adjusts posture. Discomfort? Switch back to cardio. ( swimming is cardio but lot of repetetive wrist action, i wouldnt recommend it )
Once rsi or persistent discomfort:
stop, try to bypass a regular doctor and go straight to a specialist. I have been tagged 2 times with rsi. One time it was displaced muscle in my hand because of boxing impact ( but it's something you could get holding a bike pedal driving over a rock ) and second time was a knot stuck in my neck. Simply ignoring the doctor advice and going straight to a specialist such as a expert fysio therapist is a good idea. Because RSI itself means your pretty much fucked, surgery is generally not good idea since there is no garantee that the pain caused comes from the place it hurts. And the damage done and strenght loss will only add to a worse form of rsi.