What I think would be on the senshi's t-shirts (remember, everyone, this is meant to be vaguely HUMOUROUS. I'm not really serious.)
Ami- "Life is short. Study hard. And study hard. Did I mention study hard?"
Minako- a Sailor V t-shirt. Failing that, a Bart Simpson t-shirt reading "I didn't do it. Nobody saw me do it. You can't prove anything."
Usagi- One of those Homer Simpson t-shirts that go "Mmmm... donuts." Alternatively, a shirt with a big picture of Mamoru's face so she can be close to him even when he's in America ;-)
Rei- I'm really tempted to say she has a brand name shirt -- "A Mars a day helps you work, rest and play" but it wouldn't be very Rei-like. Probably her t- shirt says something like "Feeling hot, hot, hot!"
Mako (or more specifially, Lita)- "I'm too talented for this shirt." (Bonus point for anyone who gets that before I point out that the shirt's a size too small.)
Hotaru- "Life's hard and then you die. And die. And die. And die. It's not really fair, is it?"
Haruka- "I don't brake. For anyone." (then, in little tiny letters "except for Michiru.)
Michiru- "Protect our oceans." (then, in little tiny letters) "You think I want to throw polluted water around?"
Setsuna- "Time is relative. Your job is relative. Too bad I hate my relatives."
Mamoru- "Roses are red, violets are blue. Throw flowers around, you'll be amazed what they can do!"
Seiya- "Some day my princess will come... well, ONE of them is bound to like me, anyway..."
Taiki- "Fight illiteracy." (I figure only Taiki would write that on his shirt and wear it in public, trying to make his statement to those who can't read.)
Yaten- "If you can read this... you're way too close, and in my personal space. Get out."