Let's see... some of my favorite t-shirts:
"Does the name Pavlov ring a bell?"
(This one from a rescue group):
A cartoon of a dog with a teenager on a leash at a shelter, the caption reads: "Rover returns his owner because he disobeys." The dog is complaining, "We even sent him to obedience school."
"Many are chosen, but few come when called."
(This one isn't a 'dog' t-shirt, but I like to wear it at agility competitions
"I don't know where I'm going, but I'm making good time."
I created this t-shirt for our agility club as a fundraiser:
"Agility is a contact sport"