Dear nanekitoo,
I don't know where I first heard this but I have tried it and it works WONDERFULLY!. Give your dog a few tablespoons of tomato juice. For some reason this apparantly reduces the urine's acidity and it will no longer burn the grass. (that explaination was to the best of my memory) Anyway my dog loved the tomato juice treat and my lawn stopped dying and and large yellow patches. Good luck.