Neither Twain nor Shakespeare are protected by copyright in this day and age.
But lets say you used someone a bit more modern. A phrase is probably not enough of an artistic work to be protected by copyright. You don't have an issue there. So if you wanted to make shirts that say "I have a wide stance." quoting Larry Craig, he probably couldn't do much.
The only run in you would have with IP laws is trademarks. If the phrase happens to be the trademark, your mark could infringe directly or by dilution on that mark if you put something like "Good to the last drop..." on a T-shirt.