Love this web site: http://www.itsapreemiething.com They have tons of information and inspiration along with some cute and clever clothing items for sale.
This is my personal favorite: "O2 (oxygen)...it's not just for old people."
They also have onesies and t-shirts printed with:
"It's a Preemie thing"
"I've earned these rolls" (of fat--so true!)
"I'm older than I look"
For Twins: "Baby A" and "Baby B".
They don't have this one, but I've always wanted to have parties and t-shirts for Preemies when they reach their due dates celebrating "I'm finally ZERO!"