Which of your photos to start from
Most shops already have one of three photos for every garment: a ghost-mannequin shot from a studio, a flat lay on the floor, or a phone picture of a dress form by the window. All three work as input. They are not equal, though, and picking the right one saves more regenerations than any prompt trick.
If you have a ghost-mannequin photo, use it. It shows the garment in three dimensions: the neckline holds its shape, the sleeves have volume and you can see where the hem sits, so the model fitting it onto a body has the least to guess. A flat lay comes next. It shows the outline and the print clearly but hides drape and length, which leaves the model to decide how a skirt falls or how far a jacket drops below the waist. A dress-form photo sits in between, with one advantage of its own: it already looks like your shop, so the boutique can stay as the scene.
Whatever you use, get the whole garment in frame, evenly lit and not folded over itself. A hanger shot where the shoulders twist, or a flat lay with a sleeve tucked under, hands the model a wrong outline to copy.