Reduce the protein. :) The ideal recommended by very experienced breeders is to gradually reduce the protein content of her diet from when she comes into season until she's having about a third of her normal meat ration replaced by broth-soaked biscuit. Adding two raspberry leaf tablets (I use the ones made by Denes) twice a day can also be very helpful. Give her lots of hard exercise and keep her busy. Don't give her time to mope about and get broody, but be very public-school 'jolly hockey sticks' with her.