I would start with a metronome at around say 110 BPM, and just do alternate picking, straight 16th notes. When you get that up to about 120 BPM (for an extended period of time) start integrating some rests, triplets (the Iron Maiden gallop feel like the "Trooper"), and quadruplets and try changing between them a la trash metal. Make sure you can do the same thing with power chords and all that as that is essential in the thash genre as well.