All you metal players know that having a high level of endurance in your picking hand is key to heavy metal guitar playing. Look at some of the masters such as James Hetfield, Kerry King and Dave Mustaine. They can all down pick at lightning speed and do it for hours.
Here is a great video lesson from JamPlay.com on an exercise you can incorporate in to your playing to help build up your speed and endurance so you can down pick the masters of metal!
Be sure to check out my article Battle of the Metal Guitar Courses for a review of my favorite guitar instruction for learning to play heavy metal on the guitar.