RSI from typing is typically caused by excessive wrist extension, though wrist pronation, ulnar deviation, etc. can also be bad. As can any other un-neutral body position held for a long time while working.

The way to prevent it is to type with a posture such that all of the joints of the arm/hand (not to mention neck, shoulders, etc.) are in a neutral position with as little static load as possible.

This can be accomplished more or less on a standard keyboard, but is much easier with a keyboard that has a split halves, tented. It is also helpful to type with a light, bouncy style, which can be helped by careful choice of keyswitches.