I found this article through a doula's blog, and I think it's a really good one.
Birth affirmations are mantras or positive quotes or sayings that you can repeat to yourself during pregnancy and childbirth to help put you at ease and focus during labor. When I observed a Hypnobabies course, I found that one of the CD's they give to expectant mothers has pregnancy affirmations. One track is for during pregnancy, and the other is for the birthing day (when the mother is in labor). I have listened to the pregnancy affirmations track, and I love it. It's extremely comforting and positive.
I've not tried using affirmations while in labor, but I can imagine how powerful it would be, especially if you have been practicing throughout pregnancy to prepare yourself by using positive affirmations.