Sometimes bathing even with a high brand dog shampoo, can make matters worse. Are you sure that it's not some type of skin infection?
If your quite sure it's just dandruff, I've got a few questions:
What do you feed?
Do you supplement with anything?
Shampoo's, oatmeal anti-itch shampoo etc. can sometimes help hide problems, but I've found with my guys supplementing has worked the best. They get herring oil everyday, the "dosage" depends on weight.
The reason I asked if your sure it's not a skin infection, is I went through this with my bitch. She had terrible large chunky flakes appear quite quickly, a serious change from her normally silky, shiny black coat. I immediately went out and bought the top of the line Oatmeal anti-itch shampoo, along with the best conditioner and a vitamin spritzer. Didn't work. I went out and bought an even higher end brand of supplements. After a week, no improvement. I ended up eventually going to the vet's and it turned out she had a skin infection.
So, if your in any way unsure, I'd pay the $30 for the vet's to rule out an infection. And then start deciding on if you could feed a better food, supplement some Omega's or what not.
Edit: Just wanted to add that often, in the winter, no matter how much you supplement, bathe, condition etc. your dog will just get dandruff. Like people, their skin gets dry and flaky.