I use Pellon 911FF fusible interfacing. I put it on the backside of the T-shirt, and then I dampen another plain T-shirt to place on top of the non-fusible side of the interfacing. It makes a really nice pressing cloth.
I usually buy it at Jo-Ann's on sale for 1.49/yd. I can get two 15" T-shirt blocks out of a yard of interfacing.
I really like the results I have gotten with it so far, but I've seen wonderful results with the woven fusible interfacings too.