I find this one very inspirational:
"I'm the type of a person who's the type of a dog who bites back." - Mike Katz
No offense to you or your taste, but I always thought that quote was a little retarded. It's just cumbersome as hell. He should have just said "I'm that/the dog who bites back", or at least "I bite back" if he wanted to keep that dog vs person separation

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But one quote I've always liked was from Kevin Levrone in Full Blown. It's towards the beginning when he's doing cable crossovers, and he just looks into the camera and says "IT'S ABOUT BODYBUILDING" while cranking out a set. Not the most mindblowing thing ever, but for some reason that gets me really pumped to train.