You can purchase fabric markers from craft stores but thus far I haven't found a brand that outshines Sharpie.
I like to hand draw my own clothes and occasionally sell my designs, mostly because I would rather wear something no one else has, something that has personal meaning or humor to it.
If you're not really "the artistic type", even hand writing your favorite quote or perhaps your own catch phrase is a great alternative. Think about placement, somewhere unexpected, like the bottom corner of the shirt, or the sleeve...use your imagination.
[artist Kat E. V. Day]