Some women describe a Ring of Fire during the final moments of birth. At this point, you body is saying 'slow down'. Pant, actively prevent pushing, holding back until the fire is out, may avoid a tear. Pushing through the fire may result in a tear. This involves being VERY aware, and being supported not to rush. It is helped by not being on your back or in a squat, not having an epidural and not having coached pushing.
When women are instinctively birthing, they tend to go to a hand and knees, or on knees leaning forward, position. They will hold back and breath, slowing down and allowing their body to stretch before continuing. If you have birth before, you are more likely to do this effectively.
Perineal Protectors: Midwife Thinking is an indepth look into the evidence surrounding perineal care. Covering the antenatal interventions (such as massage and 'epi-no') and some in birth interventions of various invasiveness.
If you have had an augmented labour, have an epidural and are birthing on your back, some of the perineal techniques described by 'Midwife Thinking' may be useful. For example, a warm compress has been shown to reduce major tears, especially in first time birthers. It may also help if your attendants encourage you to slow down, and not push through. Gloria Lemay in the Midwife's Guide to an Intact Perineum, describes what the pushing phase of birth is like, the thoughts you may experience and how your attendants can best help you.
This conversation piece gives these insights (see the article for links to the research):
Perineal trauma is more likely:
Perineal Stretching Massage by Caroline Hastie describes the massage technique, but the caveat is this: YOU DO NOT NEED TO DO ANYTHING. If it feels good for you or helps you to become familiar with your body, go for it - with knowledge and respect.