Vinegar, get some white vinegar and but a table spoon in about 500ml, put through your hair and rub your scalp. then wash your hair normally...
everyone has dandruff, just some people get big clumps of it for various reasons... the vinegar helps break down the old skin on top of your head (because of the acid in it)
You can also try stop washing your hair all together, just google "no poo" and you might find some sites about it (I havent washed my hair in about 3 years and my hair is the best it has ever been)