I love this technique! Super cute making a Halloween Batty shirt for the kids! I was thinking of using a stencil and "reversing" the technique as it were by splattering fabric paint on a white shirt (say to make stripes for a jailbird).
Cheap t-shirts and a bleach pen. Flat surface (a magazine or something inside the shirt), make your design, leave for 20 minutes or so, rinse in the sink with soap and water until completely rinsed out. . . I always wash in the washer afterwards.