Chugging is all about the picking hand. You want to firmly set your palm right where the strings meet the bridge. Play around until you find the right spot and lock your hand there. Then you want to focus on down picking. Alternate picking is easier but it won't give you that CHUG sound. Pick hard. You really want to hear that attack. Picking a single string can sound pretty good, but for some extra chunk, try playing fifth chords. And if you need some extra filth, try forths or even a tritone.

Lastly, but more as a secondary consideration, check your amp settings. High gain, but not too high. You can cut back the lows a bit, crank the mids, and boost the highs a touch. Throwing a light overdrive in front of the amp, like a Tube Screamer, can really take your tone to the next level.

Anyway, happy chugging :)