Just posting quickly and haven't got time to write a long post.

But have a look at this website:

Big Baby

Also this article on the main site.

Perineal massage is really worthwhile doing, but you have to start it within a reasonable time frame (around 34 weeks) and make a commitment to do it religiously, every day. It can't work if you only do it once or twice a week

Birthing positions are also important, upright positions generally result in less tearing. Also get a hold of a copy of the Pink Kit. It has loads of information on birthing positions and avoiding tearing.

The biggest deciding factor in whether or not you will end up with an episiotomy is, believe or not, nothing to do with the size of your baby. It's the practices of your OB. Some OB's have ridiculously high epi rates and all the peri massage and active labour in the world probably won't protect you if yours is one of them. But since yours is recommending peri massage etc that is a good sign that he/she isn't too "snip" happy