People have been using the word dandruff as a catch-all for anytime you get flakes in your hair, but just because you have some flakes doesn't necessarily mean you have dandruff. Dandruff is large greasy flakes while dry scalp is the little white flakes in hair most people think of when they talk about dandruff. In the ten years I've been cutting hair, I've only seen actual dandruff in someone's hair two or three times. Now that we've got that out of the, I was reading a good article on GQ about habits men have that are causing their dandruff. I agree with what they wrote and don't really have anything to add to their list, but there are some environmental factors that you can't control that can affect your scalp. Except for the weather we've been have recently, Los Angeles is generally dry, and I know a lot of guys that started noticing flakes in their hair after moving to LA. The changing of the seasons can also do a number on your scalp. Even though we don't get much of a winter in LA, I do see more flakes while cutting hair this time of year.
If you've followed the advice in the GQ article, and still have flakes, it may be time to switch to a different shampoo and conditioner. I have read good things online about Nizoral AD, but I don't have any experience with that particular product. What has worked best for me is Aveda Scalp Benefits and the Eufora Complete Shampoo with their Revitalizing Treatment. If you do everything in the article, get a good shampoo and conditioner, and still have scalp issues and see flakes in your hair, it may be best to go to a doctor. Below is a link to the article I referenced.
http://www.gq.com/story/bad-habits-causing-dandruff
