If you're right-handed, you want to rest your right hand just in front of the bridge, like Magnus said. If you want a lot of muting, move your hand away from the brigde slightly, and move it the other way for less muting. It takes experimentation on your own to develop a comfortable technique. But once you get it down, you'll be playing those thick & chunky crunchy riffs like there's no tomorrow. Palm muting is also good for keeping strings quiet when you aren't using them.