Garlic or brewer yeast  

I would say brewers yeast. I used garlic for worming the dogs, and also it is great for repelling mosquitoes but everyone has been saying that garlic is toxic for dogs. Brewers yeast not only will repel fleas, but it is also great for helping control arthritis and other physical problems in dogs. The only thing about brewer's yeast is that it will give your dog gas. I combat doggie farts by giving them yogurt daily. I buy the least expensive and they love it. I give my pit/lab mix one of the individual serving containers daily and it has sweetened up the house enormously.

Unfortunately the pit/lab really took it to heart when someone told her that there needed to be smell in dog farts. She can evacuate the Empire State building with one of her farts!!

If you are going to the trouble of giving your dog brewer's yeast, you also need to take care of the house and vacuum. I sprinkled borax on my carpet and worked it in with a broom. It dries up the eggs so that they don't hatch. Also after you vacuum you need to get rid of the bag as it sucks up the eggs, larva, and fleas. You can put either moth balls (I hate the smell of moth balls and wouldn't have them in my house) or use some of the borax in the bag before you vacuum. I hadn't thought of the vacuum bag until I watched a program about fleas and the vet said that all the flea prevention in the world isn't any good simply because people don't think about the best breeding ground for the fleas, the vacuum. I started putting borax in the bag and it cleared up that problem.