A good t-shirt design is like a haiku. You have to figure out how to say what you want with a limited amount of space, colors and words....It took me a long time to figure that out though because I don't really wear t-shirts with graphics on them hence I never really thought about how to adapt my work for that medium. When I was asked to submit some designs for a t-shirt project a couple months ago I really had no idea where to start. I submitted my painted illustrations point blank with a bajillion colors and no real message and a couple of other super (in my mind) "t-shirty" ideas but I really didn't know what I was doing. Luckily out of that messy bunch they accepted a design BUT rejected everything else. After that I started observing t-shirts more carefully; I may not wear them but sometimes I'm called to make them....so I must study. The best ones are simple but clever like a haiku. Anyways I guess the pointed observation has been paying off in little ways because now I get random t-shirt ideas all the time....
Oh, and it's rare but there are t-shirts that I do like. I'm drawn to stripey t-shirts, some trompe l'oeil stuff and t-shirts with photographic images on them. :-)

(this is cute and falls into my photographic image category)

(this one though it falls in the photographic image category not so much....although I do like the shape of the t-shirt better than the other one)