There are a few things you can do (though some things are outside your control like the position and size of the baby - a baby trying to be born with a hand by it's face will tend to cause more tearing than a baby who is only head-first).
1. Perineal support by youur OB/midwife during pushing - including teling you when to pause to let the tissues stretch. I had this with both births (with DS the midwife massaged the area as I pushed with oil)?) and with DD the midwife used warm compresses (washclothes soaked in warm water) on the area as I pushed - this is also called "supporting the perineum'
2. A longer pushing stage - if your baby flies out in 2 pushes then it doesn't matter if it is a preemie who only weighs 4 lbs - you're at a higher liklihood of tearing than if you push for 30 minutes which gives everything time to stretch instead of tear (think of this like silly putty or taffy - if you warm it up and then gently stretch it you can get it MUCH longer than if you grab it cold and yank where it breaks. Similar analogy to ours skin - having your skin warm, lubricated, and slowly expanded will reduce your risk fo tearing)
3. Research different pushing positions - I have read that side lying has a much lower rate of tearing than other positions. Also, only push when you feel the urge (unless there is some fetal distress complicating the process). working with your body's process instead of outside coaching will hep things to go more smoothly too.
Mom of 4: Boy (10), Girl (7), Boy (4), Girl (2)