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

What really helped me improve my typing speed is...when I learned to type I was in H.S., and we had manual typewriters with all the letters painted over. Our only reference was the big keyboard poster on the wall. It was hard, but I memorized the letters and finger movements, then, practice, practice, practice.
Not being able to look down to see the keys was probably the most helpful part of learning.
Now its 30 years later, and it's much different using computer keyboards, so I am learning again.
Wish me luck!!