When my fingers hurt after too much time on the keyboard, I...take a break.

Accuracy. The foremost reason for typing mistakes is because the typist is not 'seeing' the keys with their fingers. If you're one that types 70 w.p.m with only 90% accuracy (which is rather poor), slow down to 50 w.p.m and start paying very close attention to the keys you're pressing. When you make a mistake, pause for an instant and make note of where your finger position is so that you know why you made the mistake. That way, when you come up against a similar letter combination, you'll think about where those keys are and type the combination in correctly. Slowing down to 50 w.p.m is arduous, making you feel like a weakling of a typist, or for those that are 50 w.p.m typists, slowing down to 30 w.p.m will make one feel the same. But cutting down one's w.p.m to, around, 65% or 70% of their top speed to focus on accuracy will bring about an increase in top w.p.m by 10% rather shortly.