Flamenco star Adam Del Monte gives a lesson on classical guitar tremolo. A nice video lesson on tremolo largely discussing pre-planting the right-hand fingers (useful in all right-hand guitar technique).
Overall a great lesson, just keep in mind (especially my students) to keep your guitar position in the classical style and try not to bend the wrist (to align the knuckles with the strings) too too much. I like how he divides the different aspects of practicing the technique (control, speed, tone, etc) as well as giving each finger (p,i,m,a) individual attention.
Video Link: http://youtu.be/XrDTTdE8Rig