I've been playing for 14 years and still do this exercise. After I go down and up once, then I move up one fret and keep going until I reach the 12th fret. Another good tip is try to keep your fingers as close to the fretboard as possible so you don't waste time getting them back to the fretboard. You can also try variations of this. Instead of 1-2-3-4 you can do 1-4-2-3 or 1-3-2-4 or whatever you want. I believe Papa Gates talks about these in the Intro to Alternate Picking and the lessons right after that.