It really depends on the cause, I wouldnt try any treatments at home until you have a diagnosis. Did the vet take scrapings to check if there were any mites causing the skin condition? If it is an irritation purely on the abdomen it could be caused by contact with an irritant on carpets or floor (bleach or shake n vac type irritants) Is the area itchy, if so then a hydrocortisone cream would help which you can pick up from the chemist but cannot be used long term as it has steroids in it and will cause thinning of the skin. There are too many causes to think of and these things can get worse if left untreated, if you notice smelly discharge or weeping then it may be infected and antibiotics will be required from the vet.