You can make a quick flea shampoo by adding a small dropperful of Listerine to your regular dog shampoo. Like any flea shampoo you will want to make sure you keep it out of the eyes and don't let your dog eat the lather.
(I once had a dog named Yushi who was adamant about eating shampoo lather, so much so that I had to tie her muzzle with a nylon stocking to bathe her).If you use a topical flea treatment it will make it somewhat less effective. I don't recommend using flea shampoo of any type, natural or not when your dog is wearing a topical flea treatment. Just use a gentle sulfate free dog shampoo.
I have used a homemade coat spray at ringside when showing shortcoated dogs like Dobes and Salukis that consisted of a half teaspoon of Listerine added to a 15 oz spray bottle of water. After spraying, a quick rubdown with a genuine chamois cloth brought a beautiful luster to the coat. For double coated breeds like Corgis, I have used distilled water in the spray bottle and then combed the hair backwards after a light spray remembering to avoid the eyes.
But my favorite use of Listerine is as a cheap yard spray to kill fleas. Mix one ounce of dish soap, a half teaspoon olive oil and a tablespoon Listerine to a gallon of water, then spray the yard. I prefer phosphate free dish washing liquid. We used this mixture for years and years to keep fleas at bay in the yard. As an added benefit, the thymol in Listerine helps repel mosquitos to some degree too.
And you thought Listerine was just a mouthwash...