We recently moved from a fairly bug-free high-altitude place to a new state where we were warned: there are lots of bugs. Sure enough, the dog we've had for 10 years without so much as a fly on her all of a sudden had little things crawling on her. The vet confirmed it was fleas. And this was after very strict applications of a best-quality flea control. We sprayed lemon juice and water, and got some all natural lemon-grass and rosemary sprays, but it didn't help. Then we tried Diatomaceous Earth.
EXTREMELY IMPORTANT: there are two grades of this stuff. One is "pool grade" and is used in swimming pools (I don't have a pool so I don't really know what it does, but that's not important). The other is "food grade" or "human consumption" grade. Don't buy this stuff at a pool or landscape supply place. Buy it from a specialty pet supply store (I just typed in "where to buy diatomaceous earth in ______ [my location]). It is safe, and it causes the fleas to basically disintegrate.
We rubbed it on the skin of our dog. Then we sprinkled it on the floor of the room she sleeps in, and on the bedding. We kept everyone out of there for a day and then vacuumed (with a cut-up flea collar inserted into the vacuum bag or collection bin). I also sprinkled it outside the windows.
Within one day, there were no fleas. We feed her a small (1/2 teaspoon) amount every couple of days and just give her a belly rub using a small amount of the stuff (it's like powder).
It wasn't expensive. Just don't forget: FOOD GRADE, not pool grade.