I don't believe there is anything that will keep your yard completely flea free. Treating your yard is one of the best methods of controlling a flea population. Professional help is usually needed if you have a bad infestation.
One of the most common reasons pets get fleas is that they have dry, flaky skin, which makes it easier for the fleas to get a foothold and is easier for them to obtain their meal of blood. If the animal gets adequate exercise they will not have as many fleas.
Sweet Smelling Fleas Shampoo for Pets
1 cup Castile soap (liquid)
1/8 ounce of essential oil of pine
1/8 ounce of essential rose oil
Place all the ingredients into a jar with a lid and shake to mix well. Add the mixture to your pet's bath water and the fleas will be very unhappy.