I've been wondering this myself! With DD, the midwife told me to lie on the bed to start pushing. I argued that standing would be better to prevent tearing and she assured me that I wouldn't tear on the bed.
She told me to push hard, then soft, then hard, etc, and voila I didn't tear.
My labour was about 10 hrs (from 3cm - 10cm) and I spent most of it lying down. Also, I had no pain relief.. no perineum preparation either.

I definitely think that a fast labour is likely to result in tearing, as well as bad coaching, baby's head size, and possibly epidural might increase the chances..

DD had a small head! (Just as well coz I am a tiny person!)