How do you do it?
My arm locks up in my picking hand when I play.
I try to loosen up and think I do but I still get pains.
I hold the pick in between my index and thumb, I pick with the wrist.
I don't use my elbow except to move up and down.
I believe I have proper technique.
slow is the way to go. i am just now starting to go for that whole effortless thing, also try economy picking.
If there's tension, you don't have the right technique. I'd recommend something a little different here, but it might just work. What I want you to try is as follows:
Wash the tense areas under warm water. This will greatly loosen the muscles. Learn how these muscles feel when they are relaxed. Try to emulate how this feels when you play guitar. If you get pains even when you're playing completely tension free, that could be a bad sign. Try to play without tension for a while; if it persists, a visit to the local doctor might notbe a bad idea.