Watch out for tension, the claw, paddle fingers, tea cup pinky. Stay loose, and keep an eye on the shape of your hand. Making repeated mistakes? Slow down and make sure you are in position.
Let your fingers do the walking! As your small, untrained muscles get tired, larger muscle groups get recruited automatically; watch out for this. If your whole arm hurts, you are using way too much muscle.
Watch my examples and note that my hand is virtually motionless, all the action is going on from the knuckles down. I'm not pulling on my strings, rather striking them with just enough power and at just the right angle to control my tone and volume. This takes time to master, remember that repetition is the picker's friend!