I've been trying to refine my string skipping and playing fast licks skipping over a one string gap. I've look around the forums and have been trying to focus my picking onto the pivoting of my wrist and a little bit of thumb+index. I am resting my arm on the guitar, yet my upper arm still gets sore quickly. It feels like I'm tensing up in that area, even though I'm drawing no movement from there.

On another note, my thumb quickly becomes sore when I play a large amount of barre chords (i.e. Under the Bridge by RHCP). I'm trying to relax my hand when I barre the chord, but it seems the force needed to keep the strings down is too much. I have no problem with getting the strings down in the first place, it's just sustaining and switching them constantly that gets me.

So any exercises or tips you can help to solve these two problems? The pain and soreness usually kicks in only after a few minutes of performing those songs/licks. Anything simpler is fine.
I don't know about pain in the arm when picking but I know some tips to help with the bar chords. Get some lower guage strings and once you get comfortable with those ones switch to higher and keep doing that until you get to 13s or whatever guage you want to use.
It's not really pain, but soreness and tiredness from staying tense. I don't know why it's tensing at all.