To my knowledge with fleas only 1-5% of them are actually on the dog at any one time, so for every one flea you see on your dog there is up to 99 other fleas somewhere in their environment breeding away happily - which is why it is so important to treat the environment.
We had huge flea problems at our old house - the neighbours cats used to wander into our yard and they had a lot of fleas so we struggled to keep on top of it. We used to use frontline but since they added the second ingredient to the spot-on ones they dont seem to work well for fleas or ticks. The spray is much better but doesn't bind as fast. We use the fidos Free-itch concentrate now - seems to work very well for fleas and when I was working as a vet nurse in a very high tick area, we tested all the different products on some live ticks we'd pulled off dogs and fidos seemed to kill them the fastest everytime. And it is really cheap!