Love the Teddy Roosevelt quote!
"Just Do It" is something I say to myself regarding exercise and other things that I need motivation for. Also, I recently read "The War of Art" by Steven Pressfield, and based on this book I'll tell myself "The Resistance doesn't win today." (Probably makes more sense if you've read the book!)
Finally, if I'm sweating and hurting during a workout and hating it, I'll tell myself "Do it for your sister, because she can't" -- this one really gets me back on track. I have a sister who was in a bad car accident 8 years ago. She broke both her legs (and one arm) in multiple places, and they have healed so she can walk short distances, but it is painful and slow going. She is on permanent disability, cannot work, and basic things like a grocery shop are painful for her. Needless to say, if I start to feel sorry for myself for all the low-end squats, sweating, etc., this mantra gets my mind back on track!