Natural Repellents typically include essential oils - such as Neem, and Citronella as a spray and shampoo to control fleas and ticks; for mosquitoes, black flies, and other flying insects there's Buzz Away.
Dr. Bens Personal Insect Repellent Ceder Oil is great and a natural product to use on both cats and dogs for fleas, ticks, mosquitos, bed bugs, lice, mites, mange, roaches, ants and other parasites (This product was tested by Iowa State University at the request of the United States Department of Agriculture for the use of the United States Army in Iraq to eliminate the Desert Sand Flea problem that our troops are encountering, which chemical treatments were either ineffective or causing rashes. The test results were astonishing - this red cedar oil product was RATED far superior to its chemical counterpart. These same products were tested and proven to not only kill Mosquito's, Flea's & Bed Bugs, but is one of the "ONLY KNOWN SUBSTANCES KNOWN TO MAN THAT WILL KILL THE FLEA, ITS EGGS AND LARVEA THE FIRST TIME" Studies found Cedar Juniperus Virginiana to be Non-Toxic. Article from the Dept. of Biomedical Sciences - College of Vet. Medicine, Oregon State University - (
http://juniper.orst.edu/craig.pdf)
Also one can steep thinly sliced lemons in hot water overnight, then rinse or spray your dog with the lukewarm liquid for fleas. There's also natural flea and tick collars on the market now.
However, only 5% of a flea's life cycle is spent on your dog. So you have to find out where they hiding the rest of the time (EVERYWHERE!) and deal with the issue. Frequent vacuuming of carpets - washing of beddings, and spraying the house with a mist of oil of wintergreen helps. If the fleas are in your yard one needs to treat the outdoors with beneficial nematodes or diatomaceous earth from garden centers. They go nuts for flea eggs and larvae.
Hope that helps a bit.