Has to be the most annoying thing ever. You either have it, or you don't. It is caused by a yeast which makes the skin cells clump together.
Forget the head & shoulders adverts and zinc pyrithione, the thing that works the absolute best, is a chemical salt called piroctone olamine.
After that, anything ending in 'zole' (which shows it is an antifungal).
Piroctone Olamine is found in the Boots specialist products and in Simple. It absolutely clears your head.
It used to be in so many more products, Timotei, Dove and others, but they withdrew it - I wonder if it is because it is more expensive, or because people just didn't buy it?
Those shampoos are more expensive than most (around £5), so another way is to buy the piroctone olamine scalp treatment (also about £5) and then add a small squirt to an ordinary shampoo and leave it in, which is a cost effective way of doing it.