Flea control is as important in Bullmastiffs as it is in any other breed. Fleas are annoying to your dog, they're bad for his/her skin and they can cause your dog to leave little blood speckles all over your house and it's furnishings. Fleas are also annoying to your friends and can discourage their social side where you and your dog are concerned.
But be of good heart, modern science will rescue you. Several safe and effective products are available for flea control.
First, are the topical drops such as Frontline, Advantage and Defend. These are vials of liquid that you apply to the area between the shoulder blades and above the base of the tail usually at one month intervals. The active ingredients stay in the dogs skin and repel fleas and ticks. Those fleas too stupid or hardy to be repelled are poisoned by contact with the dogs skin.
I use Frontline. It seems to last a little better than Advantage and Advantage makes my dogs feel somewhat greasy. I haven't tried Defend in a long time. I gave it up years ago because it was very expensive. It's price has come down in the last few years.
With any of the topical drop flea controls, it is vital that you wait 24-48hrs after bathing the dog before applying. All the topical drops require some amount of oil on the hair and skin to disperse throughout the dog's coat and it will take at least 24 hours after the bath before there's enough oil on the coat for the drops to work. Otherwise all you'll do is kill every flea in about a 3" circle between the dog's shoulder blades, leaving all the other fleas unharmed.
Since I have several dogs, I can get away with only using the flea control drops on one or two dogs per month. This saves money and allows each dog a lesser exposure to the flea control chemicals. They're safe, but less is better. If you have 2 dogs, I recommend alternating the doses between the dogs.
The second type of flea control products are the "breeding cycle interrupters" such as Program. These products are given orally, usually one per month. They are ingested by the adult fleas when the fleas bite your dog. Then, the eggs these fleas lay are incapable of hatching, so the flea's breeding cycle is broken.
This type of product is OK if your dog never leaves it's home and has little contact with any animals carrying fleas. If your dog travels or you have visiting animals, your dog can pick up biting adult fleas. Program type products do nothing to fight adult fleas other than preventing them from reproducing.
Third, are flea powders and dips. They work but usually you have to apply them weekly. I find them inconvenient and messy.
Avoid flea collars. They don't work very well and they smell bad. Plus, they're hazardous to children, who may handle the collar and then stick their hands in their mouths, and other dogs, who may grab another dog's collar with their mouth, that being what dogs use to grab things with.
Collars of garlic or eucalyptus may smell nice, but they don't work. Ditto for Brewer's yeast and B vitamin dietary supplements.