Fabric, or an outfit, can be used to hide the breasts by covering them or draping the fabric over them. I have used a feather boa with good results. Sometimes I've had a model cross her arms over her chest. If, on the other hand, a model has very small breasts, they can be accentuated by using blush to shade in cleavage. There are many underwire, pushup or miracle bras that you can use too.
Some poses can also help.
I like to have a model sitting on her legs, or in a chair, with her back straight and place her hands on her thighs while leaning slightly forward. If she then locks her elbows, her arms will push her breasts together and make them look larger. I often use this in boudoir photography. A model that wants to look thinner overall can hide her body by wrapping cloth around herself or wearing full cut clothing. Have the model pose in a bathtub set, or in a bed, using the sheets to hide parts of her body.