I don't have one 
I LOVE swimming, biking, and running, and get kind of sad each time I end a training session or leg of a tri. And I LOVE the pain of working out so I'm actually happy when it sets it because I expected it. And I know with all three there's going to be high and lows and rush of endorphins throughout so I just wait for those and bask in them when they come. I love to close my eyes, raise my head up to the sun, and enjoy the moment when I run and there's a straightaway in front (i.e. track with no debris, park path with no bumps, etc.).
I have a few life sayings....
Veni, Vidi, Velcro. I Came, I saw, I Stuck Around.
Harder, longer, faster.
Anything, anywhere, anytime.
Triathlons are a huge chess/mind game. I love pain, hills, heat. Because if I suffer, others are suffering too, and one of my biggest assets/strengths is that I can make myself suffer more and longer. So I don't really have a mantra because when it gets hard I just enjoy it and look around and imagine others suffering too 
I also spend a lot of time going over form checks, etc. With swimming I make sure I'm pushing water back and not down, don't stroke too wide, get a good pull, remember to exhale under water, remember to kick, during an actual event sighting and not running into people and not getting run over takes a lot of time, etc. Biking make sure upper body is relaxed, hands relaxed, use only legs to move forward (no upper movement), pedal stroke mechanics, pull, pull, as opposed to push push, forward, back instead of up down, heel down on the down stroke, head up, look forward, elbows in and down, etc. etc. And not running into anything and not getting run over. On the run I just go because for that one the body just knows what to do after you've ran enough and I don't really have to pay attention to breathing anymore. All this takes a LOT of time either in training or an actual event.
Edited by GatorDeb 2012-09-12 6:01 AM