You get a better sound from the guitar by using only down-strokes. Your leverage just isn't as good when you up-pick. Therefore, I use as many downstrokes as possible. I developed a technique of quick picking, using only down-strokes. But sometimes, for very rapid phrasing, I have to use alternating up and down-strokes. I also hold the pick in an unusual manner-with my thumb, index, and middle finger. By picking this way, all I do to change the dynamics and volume is tighten or loosen my grip on the pick. I don't have to pick harder and my wrist remains loose.