How to stop bad dandruff?

Ever since I was about 14 (I'm 19), I started to get really bad dandruff. It seems to get worse in the spring. I've tried everything from Nizoral, T/Sal by neutrogena, different types of sulfur shampoo and hot oil treatments. I have short sandy brown hair and its shows alot sometimes and its really embarrassing. I'm african american so I wash my hair once a week sometimes twice and also lightly grease my scalp. Is there any advice you can give or what has worked for you? By the way I had eczema when I was younger and still hasn't completely grown out of it yet.