my mother once sent a t-shirt that said 'hold me, i'm a fermata" needless to say, i wore it only once (felt too conspicuous and didn't want just any stranger running up).
however, there must be some better one liners. how about a list of musical terms. then, on the back 'if you understand that, you're a musician.' ok. that's dumb.
if you're like me, occasionally i forget a well known musical term and have to look it up again. those are the ones i need on a t-shirt.