I have same problem but I find that its my left hand that is much weaker and I'm right handed. To build more stamina meaning build more muscle coordination in my arm or arms. I've started doing extremely hard songs using stepmania to make the fingers move more like how people who have piano lessons but more so arrow lessons. XD Thou if its your wrist such as I have problems with mine I have a wrist brace I wear to give more support but that its just not for typing hrm anything keyboard related I find it helps a great deal.
But I' ma die hard XD I unno if those helped or not, but those are things I find that have worked for me and when you need a rest take a rest XD!
