As for witty biting remarks, hmmm... I don't know about witty or biting, but two things come to mind that I've had to say a few times too many:
"It's simply mind over matter. Are you going to let a simple matter defeat you?"
"We can sit here all day while you make excuses. The reality is, nobody cares about excuses. Excuses don't get you anywhere. Excuses don't get the job done."
...If I hear too much whining about something I might throw in,
"Well then, you're clearly defeated. I can't help you."
It's important to note, that I'm typically very patient with people, but I have little tolerance for two things: people doing things half-ass and 'quitters'. As my dear Biology professor taught me, "there's more than one way to skin a cat." So true.