Heavy guage strings, heavy guage pick, pick at the natural angle, don't rotate your hand to strike the strings perpendicularly (this will only slow you down and make your sound thin and eventually will probably give you carpel tunnel syndrome for holding your wrist that way!) Work on isolating the stroke so that the only movement you see is with the fingers holding the pick (at first this will seem impossible) rather than moving your whole arm. A little wrist movement is okay, but eventually you can get a good strong tremolo with the two fingers and only the muscles that work them. The motion isn't so much an up and down as a back and forth. When you first practice it, break the stroke up into two parts: forward (downstroke-- let resonate), (backward let resonate). Then treat it as one stroke (forward, back-- let resonate). Then treat it as a double (forward backward forware backward-- let resonate). Then as a triple, quadruple, etc. until you can treat tremolo playing as one continuous motion for as long as possible and don't have to worry about the strokes as singles. A perfect tremolo will sound like one long note rather than a bunch of single notes-- similar to the way good violinists make it sound like one long note or fast double bass players make a rumble rather than a bunch of notes at random volumes and velocities. Then, if you want to make it grungier or nastier you have the choice of turning your pick however you want, purposely accenting certain notes, etc. so you can have control over your sound, and not vice versa.