It depends on the duration of the notes played. But usually, alternating between i and m will do the trick. The obvious difference is a triplet on the same string, usually on the same note, as in a classically played tremolo, where a, m and i are used repeatedly.
Often, even in very fast runs, just i and m are used, alternating. Paco de Lucia is using i and m in this heart pumping tune with his cohorts. Of course, he's not human. Nor are his cohorts.
http://www.youtube.com/watch?v=knghI...eature=related