To firm up your dog's stool, try switching to a high-quality, low-fat dog food that lists meat as the main ingredient. Also, if your dog currently eats wet food, gradually replace the wet food with dry food until your dog is only eating dry food, which can help make its stools firmer. You can also try adding some fiber to your dog's diet by mixing a little oat or wheat bran into its food. If your dog's soft stools persist, take it to the vet to rule out any other health problems.