I picked up a guitar 13 years ago and learned and got real good, then for the better of seven years, I didn't play because I fell on hard times, No money, ect. After getting back into the swing of things 2 years ago, mostly everything came back to me except stamina in my strumming arm/wrist, When I was 13 it came naturally, Now it's not. Any advice on building up stamina in the right hand/wrist/arm. Surprisingly I can't find much on the subject if anything. I've heard some ideas but I haven't found over the past 2 years that they've been all that effective. Any ideas?
Start with the easy stuff always.... go for the down, down, up, up, down and go really slow and once you have the jist of it then go a little faster, if you muck up go back to the previous speed get back on track and then go a bit faster again. ect.

try practicing just this for half an hour I promise you improvment.