Once surgery is done and there's no further need for surgery, position will not help much. Just ensure proper and regular dressings and maintain hygeine.

Position of sleep is not relevent.. ideally they would have used a corregated rubber drain or a gause drain both of which works on capillary action.. so position doesnt affect much...but better to keep the affected portion elevated!.. so lying on right side would keep your left side elevated.. that is one possible answer... but there was a large cavity and lot of pus (which is unlikely as such in most breast abscess) than lying on your left side for a day or so would gelp with drainage of the pus... but the best position is to lie down on your right side