1. TREAT THE PET - Whether you use expensive prescriptions from a veterinarian like Sentinel (which, by the way, prevents heart worms, too) or nape-of-the-neck treatments such as Frontline or Advantage, you have to treat the animal on a regular basis. Part of this step also includes bathing the animal as frequently as possible. Vacuuming a pet's fur is probably the most organic way to treat, when you get right down to it. I've recommended diatomaceous earth (D.E.) in the past, but veterinarian friends of mine usually warn against using it too much because it can dry out the animal's skin rather quickly.
2. TREAT THEIR LAIRS - That means treat areas of the yard where they hang out or nap should be hit with synthetic or organic insect controls. "Manic organics" will burst an aneurysm at the mention of a chemical control where pets hang out, but safety labels on many insecticides like bifenthrin (one of the best all-around flea and tick insecticides) note that they're perfectly safe for the average pet as long as the animal is not eating the granules or treated grass. Nevertheless, aside from cedar as a deterrent, there are very few "organic" insect killers out there. If a lair is used by a mostly indoor dog or cat, it can be treated with the next step.
3. TREAT INSIDE - Even if your dog or cat is mostly an outside animal, you still have to keep an eye on inside carpet and furniture where they like to hang out. The best way known to treat inside is to vacuum, vacuum, vacuum. The more you vacuum, the more insects and eggs are sucked away. Treating carpet with D.E. before vacuuming is not a bad idea either. There are also insect-growth regulators designed for inside use that are odorless and will not stain anything. Nylar (RIGHT) may be the most well-known among them, but they all prevent fleas from procreating.
Beneficial Nematodes - Usually, in this part of the state, they are very expensive and they simply have no proven track record. Since they need a moist, friable soil to do their thing, in my opinion, people with improperly raised beds waste their money applying them to clay soils. Those with organically enriched soils probably don't have insect problems because they have a healthy garden. An irony in the use of nematodes is that they need a food source to maintain their viability. So, if you eliminate fleas and their larvae, there is no food source, and the nematodes wither away.
Garlic Powder - I have the recipe for a garlic treatment you can add to a dog's food. If the animal gets hot as it plays, however, the pet may smell like a garlic. But the recipe does work.
Herbal Powders - You can try pennyroyal, rosemary, lavender, thyme, etc., but be very careful because some animals are naturally repelled by these powders, Apply them to the wrong dog or cat, and you could drive them crazy. Think about it! Plus the jury is out on them since there is really no empirical research proving their effectiveness.
Orange Oil - Good to spray under decks, for example, as a natural insecticide. As a pet owner myself, I would be very leery of using it on the pet itself. There are a bunch of home remedies out there that recommend the product, but I worry that the acidic nature of it might produce adverse skin reactions.