I would actually disagree with comparing stretchmarks with tears as my belly stretched galore during pregnancy but I only had a slight tear after birth that didn't require stitches and healed within the week.
I would agree with Flowerchild re. positions for labour...squatting/standing/any upright position opens your pelvis up to THIRTY PERCENT MORE (!!!!) than lying on your back...
I did perineum massage from 38 weeks onwards about 4 times a week and although I tore, the massage certainly helped me recognise and not freak out about the crowning feeling you get.