JSondy has a terrific point, one that a whole bunch of entrepreneurs would do well to learn! It`s the "lifestyle."
One of the foundational concepts in advertising and marketing is that
the images, music, and language often are designed to create the
"impression" of a possible lifestyle for the buyer. The classic is the
car ad featuring beautiful young women, where buying the car implies a
lifestyle of debauchery. (I love that word! Had to get it in
somewhere.

)
At any rate, tee shirts go along with the same program. Anyone can get
a t-shirt, even a plain white undershirt. But wearing a t-shirt makes a
statement. If you can put across that statement in images and language,
they you make an emotional connection with your potential customer.