montanablu has it down with the thinnest interfacing possible! I got some SUPER cheap SUPER thin stuff at JoAnn's and it's perfect for T-shirt quilts.
I usually cut the shirt in half (up the sides and across the shoulders) so I can then play with the pieces on the floor to help determine my layout. This also helps me to see how gi-huge-ic the quilt is going to be, and helps me determine how small I'm cutting down the blocks, what size sashing, borders, etc.
As for backing/weight: I've done it a few ways. The DZ one is a "rag style" - I sewed strips to each shirt to make the same size as the "back piece" quilted front to back, then sewed the squares together The black star one has two layers of batting, the brown & gold one has none. The red/black one has no batting, just fleece as the backing. Do what you like. =) If I can help, just pm me!