Its not something you can do ahead of time, persay, but there are things during labor that you can do. like hot compresses, perinal massage, oils, lubricants, not power pushing, letting the skin stretch, and water birth. Seriously, waterbirth has been the best for me! Also, avoiding the lithamoy (sp?) position,avoiding an assisted delivery, and having an active birth where you pick the position will help loads.
As PPs have mention you can do various things like massage before hand, i did that with DS1 starting about 6 weeks before EDD. TBH though, im not very confident it helped much...the girls are right, pushing a watermelon out your foof is lite-years away from stretching the vaginal area with your fingers and oil
After having two water births, i can honestly say the best effects come from the things that you do during labor, moreso than what you do prior to labor...but it never hurts to try i suppose.