You just gotta practice man. As for your picking and motions... you should pick with your wrist and manuever (sp?) with your arm. You'll build up tension because you are going fast and your hand/arm is not used to it. Once you build up strength and strech out your wrist ( I geuss you can say thats what happens) you'll be able to pick fast in longer durations, and build up speed beyond that. Also, don't forget about alternate palm mutting picking. I can be harder than regular alternate picking at first, but once you get used to it, its as cake as alternate picking.