Beauchamp, I remember your mantra list from your last pregnancy, it was really good! I wrote some of them down myself, to use in labor. And, like you, I mostly forgot about them when the time came. :)
At some point late in a veerrrrry long labor, I started rocking on hands and knees and saying with each contraction, "Let gooooooo, let goooooo." I'm not sure what prompted it - but I suspect the baby was stuck behind my pelvic bone and I was trying to convince my body to let her out. My MW was convinced I was having some psychological hangup and started asking all sorts of leading questions about reasons why I was afraid to let the baby come out. It took me several minutes of staring at her blankly before I realized what she was doing -- then I laughed and said, "I'M totally ready for this baby, I'm just trying to convince my body to get on board!"
I think a lot of it is just listening to your body and trying to use words that will help it do its job! Having a few things prepared can't hurt, though -- I'm sure I used some of my mantras during labor and just don't remember.
My mother says I spent a bit of time in transition saying over and over, "Hypnobabies LIED!"