I have started feeding my cats Science Diet Hairball Control canned food and have noticed that my girl cat who is notorious for having white flakes hasn't had it as bad since I changed their food.
JC had very bad dandruff a few years ago, and the vet gave me some capsules (I forget the name, but it was a German product anyway, and didn't help). Since I rotate foods, I finally figured out that the problem only occurred when he got dry food containing dried beet pulp. I stopped buying that particular brand, and always check to see if any potential new brand contains beet pulp, and the dandruff has disappeared. So you might be looking at a food allergy.
One of the best things you can do for dry skin and dandruff is Omega 3 fatty acids. You can buy foods that tell you there's extra fatty acids in them, or you can just buy suppliments to put on the food. You can get the suppliments at most pet stores, and I have even seen them at Wal-Marts
I've remembered the name of the product: "Lupoderm". Sandie's tip is a good one. Get the 500 mg gelatine capsules. You can puncture one a day and squeeze the oil directly on to the food for about three weeks, and then try once or twice a week for "maintenance" after the problem clears up.