Some shedding of skin is normal for them, as with us. When the air is dry in winter, they can dry out the same way we do. Mine seem to appreciate a good, thorough brushing more when the air is dry (it seems to just feel good to them). If you're already brushing regularly and still seeing an excessive amount of flakes, or the skin under the fur just seems scaly, it is a sign something's wrong. Dry skin can be an early sign of a thyroid or other problem, so I would start with a check-up so the vet can make sure there's not something going on inside the body. Here's an article about why an exam is a good idea to get to the bottom of excessively dry skin:
http://veterinarynews.dvm360.com/dvm...l.jsp?id=46548
If there's no underlying health problem, switching to a very high-quality (super-premium) food may help (or to a different one, if you are already feeding that quality of food). You've got to give it about 3-4 weeks on the good food. With one my dogs, Natural Balance's Fish & Sweet Potato solves that problem (as well as an occasional tender tummy..). Her skin gets dry and her coat gets dull with other types of protein, so we are stuck with this stinky food for her. *sigh*
Fish oil capsules (NOT cod liver, just regular fish oil) also may help. I give them to my dogs with their food (mine like them and will deliberately pop them in their mouths to taste the oil). In a young large-breed, we used to feed 1-2 capsules per day. (Now that they're older, they get 4 a day because the fish oil also is an anti-inflammatory, that helps with arthritis.) I buy the great big bottles of fish oil capsules (for humans) at Costco.