I've found this site to be a life-saver: http://www.healthy-dogs.net/fleas.htm

We have three larger dogs and live in Florida where fleas are HORRIBLE. They thrive in the sand, especially at the beach.
We used to waste our money on those flea medications and such but this year we decided to try the natural ways.

We use the table salt over EVERYTHING method and the tea tree oil in their shampoo.

We left the table salt over everything over night and vacuumed up the next day.
We bathe the dogs in their shampoo with tea tree oil every week. I haven't spotted one flea since!