Do the shirt sign as you put on your baby's shirt. We make a game out of dressing and undressing by making the signs for each item as we put it on and take it off. Playing the game seems to decrease fussiness around changing clothes, perhaps because it helps give our baby some way to have some say in what is happening.
We can introduce the sign for shirt early on. As soon as our babies demonstrate interest in dressing and undressing, we can sign shirt contextually and also by pointing to a flash card.