I've made similar patterns but instead of duct taping directly over a t-shirt I first wrap the model's torso (or whatever body part you need--for instance this is a great way to make moccasin patterns) in plastic wrap. The tape adheres to this--I think it allows it to be cut and removed more easily resulting in a slightly more precise fit. It can also be, depending on one's relationship with the model, somewhat more amusing.

Moosedog