
To be honest apart from getting the bitch fit and the underlying muscles firm there isn't a lot you can do about the undercarriage, nature will reduce them or sometimes they are still rather obvious and glandular.
My Jozi has been spayed for a year yet still has obvious mammary tissue and fairly large nipples which would probably show if she were short coated.
Her daughter who had her first litter never got baggy or even looked like a nursing bitch except for the nipples, and was as tight underneath as before a litter after she stopped feeding, and the nipples have also reduced in size, though not quite to their original size, becoming flatter.