I've noticed that to play good, consistent sounding tremolo, especially on short, thick strings, you can't just move the pick back and forth because the sound is inconsistent - the string doesn't respond fast enough. What I've found is you either have to tilt the pick side ways so you need less force to pluck the string, although this creates a scraping sound on the strings, which could sometimes be a disadvantage, or you have to move your hand in a semicircle motion, which is difficult to do fast. You could also tilt the pick with each stroke so that it comes at the string at an acute angle and slides on the string rather than getting caught on it.
What technique do players typically use?
Two bits I'm trying to play: