Guitar lessons: (opens in new tab)Marty Friedman (opens in new tab)'s preferred 'spider' fingering for powerchords, uses four fingers to allows him to play faster and cleaner chord changes.
The first/third finger shape is the obvious fingering, but if you add in a second/fourth finger shape you can cross the strings and cover the fretboard swiftly.
Try alternating between the two shapes shown here, paying close attention to the fingering; as you gain confidence, try moving up and down the neck.
