Forearms for me. Best advice I can give for young guns is start preventative breaks/stretches/exercises now. I saw it coming and pushed myself over the edge anyway. I've gotten to a pretty good place with a kinesis keyboard (the cheaper split one ) and some custom foot pedals for Ctrl/Alt/Shift, but have never been the same as before.

Also, lift weights. Squats/deadlifts/bench press. Hasn't helped my arms much but been a game changer for back and neck pain.