It probably wouldn't hurt to start now, but from what I've read it's in the last few weeks of pregnancy that a certain hormone kicks in which helps with perineal elasticity. (that's why the instructions for EpiNo say not to start using it til 37 weeks I think)
Like someone else said, some people are much more prone to tear than others. Apparently it does have a lot to do with the degree of natural skin elasticity you have. I've also read that people who have a lot of stretch marks are more likely to tear than those who are blessed not to have any, as this is an indication of skin elasticity ITMS.
I've actually been reading an Ina may Gaskin book recently which has a chapter about the "Power of the Perineum" it talks about how many of us are scared of our bits being "damaged" by birth and interestingly compares it to the fact that mens private parts engorge and grow much bigger without any damage (and it's a given), so women need to have confidence that their vagina's can do this too. She suggests at the time of crowning, focusing on this part of your body and believing in it's ability to engorge with blood and stretch. Apparently if we focus on this area (one woman said over and over in her head "I'm gonna get huge, I'm gonna get huge)and it engorges with blood (as it's mean't to) it definately helps the area to get bigger and stretch without sustaining as much damage, whereas if we're scared, more damage is likely. Interresting concept, I thought.