Whenever I'm playing sitting I get a kind of soreness in my right tricep/around that part of my arm. However when I stand up the soreness tends to go away. I've been trying to find a more comfortable way to pick but I'm not even sure what is causing it. I keep my arm unanchored, keep my shoulder down and keep my pick centered between my two pick-ups. Any help is appreciated in helping me in getting this pain away.

Ibanez S520EX
Gordon Smith electric guitar
Zoom GFX-707 Multi-effects
Crappy starter amp
i have a similar problem when playing sitting down. i found it best to simply practice standing up all the time...since you'll rarely play in front of people sitting down any way (unless u do classical typ. but you shuold try the classical postition like ibason said, for sitting.