Well, I'm sure you've heard of Alternate picking, right? Up down up down up down ad infinitum. In addition to that technique, there is also sweep picking, which is upstroke up ascending strings and downstroke down descending strings. Economy picking combines the two, using alternate picking on each string, but sweep picking to change strings. Let's say you have a three-note-per-string run up the Am scale. On the low E you'll play 5-7-8- picking Up down Up. Normally with alternate picking, you'd go to the next string and use a downstroke, which most of us naturally do to stay in sync and whatnot. However, it's faster to sweep those two strings with a second upstroke, and the pattern would continue. Honestly, the hardest thing about it is breaking the habit of alt. picking, but all you have to do is start it slowly, which makes habit-breaking simple. Keeping it smooth just takes practice, and as with anything guitar, honestly, your left hand is probably going to slow you down more than your right.