Or to yourself during whatever workout you're going through?
I don't talk loud enough to where people will think I need to be in a padded room; I have to keep myself motivated. Music helps A LOT.
But when my music is not enough, I say things to myself like:
"How bad do you want it?" I'm talking about my competition goals/aspirations. Often when going to failure, and during things I hate doing. Like incline chest flyes, stiff legged deadlifts, dumbbell bench presses, spider curls. Things I have done that I don't care for: diamond push ups, burpees, single leg stiff legged deadlifts, conventional deadlifts, and body weight exercises because I have to go beyond 30 reps.
"One more rep, it's just one more rep!" ESP when I'm at the tail end of my workout, and just about every set of exercise is to failure. As well on the leg press, moving 400+ lbs for multiple reps/multiple sets is an accomplishment in itself, but it sure as heck isn't easy; and at times I dread it. :/
"It's just me and the weights." My lifting draws an audience. -_- Not bragging, just saying. Sometimes it throws my focus off and I have to get back in the zone. I have to pretend I'm in the gym by myself.
"YEAHHH BUDDY!!!" This one I keep in my head, haha! I love Ronnie Coleman.
"I'LL BE BACK!" When I'm leaving the gym... This one is also never said aloud. Oh Arnold, why are you so amazing?!
"There is no such thing as pain." Mostly during ab work.
"I will not stop" or "I will not quit" or "I got this!" Reserved for squats over 10 reps, and sprints.
There are others, but these are my staple statements, haha.