We've had 2 occassions when a couple of fleas attached themselves to one of our dogs. I phoned my homeopathic pet supplier and she recommended DE (diatomaceous earth)
We already had some food grade DE since it's useful for parasite control (a little mixed with food or in the water)
We rubbed (massaged) the DE powder into the fur all over, left it on for 3 days and after that brushed him out and then gave him a bath. Also, after vaccuming the house, we sprinkled a little DE around the furniture and bedding.
If fleas are a common problem, do the above once month. DE won't hurt but will actually help cats and dogs.