I have a short-nosed, long-back dog, but just rely on ninja-like reflexes to avoid getting snapped when doing more delicate grooming, like removing burrs.

For general brushing and bathing, I just keep him on a leash so he doesn't run off.
Working with flat-faced dogs who can't be muzzled, sometimes a shoestring is used as a makeshift muzzle. Other times, the person doing the restraining just wears heavy leather gloves that we also use for restraining cats.