Strange as it may seem, the most important part of flea treatment is not actually treating your pet, it is treating the environment.
The reason for this is that it is only adult fleas which live on your pet, while their eggs grow in carpets, bedding, the garden, skirting boards, basically everywhere else that your pet goes.
The upshot of this is that even if you get all of the fleas that are currently living on your pet, it won't be long before they get a new infestation from the new fleas growing to maturity around them.