Heres where a bit of drawing is required. With quick mask still active, using black at 10% opacity and a large soft brush, I added shadow to the inner areas of shirt to simulate the hollowness of the empty garment and create the illusion that its being filled out by a body. I use only the edges of a large brush to keep the shadows very soft and build up the density with multiple strokes for better control.When adding these shadows check to make sure they conform to the direction of the light source. If the marching ants are distracting, you can toggle them off and on with Control + H in PS.