The shampoo and conditioner industry brings in $6 billion a year, but as Rachael's style buddy Kyan Douglas points out, many people aren't spending their money on the right products. From which type of shampoo you should use to how much and how often you should use it, Kyan shares his shampoo dos and don'ts:

Dry scalp: Kyan says that anti-dandruff shampoos work really well for dry scalps, but if you notice you're still getting flakes, try showering in lukewarm water. "Don't use hot water when you wash your head because the hot water will dry out your scalp," Kyan advises. He also suggests using an olive oil treatment on your scalp once a week. "Massage it in to the scalp and then shampoo it out afterwards, or sleep on it overnight."

Frizzy hair: Use a deep conditioning treatment once a week. "Shampoo your hair, then put in the treatment and leave it in for 20 minutes," Kyan says. "You might even wrap your hair up in a towel because the heat will help activate it, and then rinse it out completely."

Dyed hair: Use color-enhancing shampoo and conditioner rather than just color-protecting products.

Sensitive scalp. Use a sulphate-free shampoo. "Sulphate is a very common ingredient in shampoo but it's actually irritating your scalp," Kyan explains. "It's such a great detergent, it creates such a great lather and not everyone is going to have a problem, but if you do have a sensitive scalp and you notice it's dried out, try a conditioner and a shampoo that are sulphate free."

Should you rotate shampoos? Kyan explains that the idea behind rotating shampoos is that if you use the same product again and again in your hair it could build up. "You could rotate your shampoos," Kyan says, "or you could use a very clarifying shampoo once a week just to take that product out. The general rule of thumb is that if a shampoo is clear it's more clarifying, and if it's very cloudy it's probably more emollient."

How much product should you use? Kyan explains why he recommends shampooing twice: "For every single hair follicle you have there is an oil gland attached to it." He says that most people use a tablespoon of shampoo when they should be using only a teaspoon to a teaspoon and a half. "On the first shampoo, notice how much lather there is - if there's not much lather that means you have a lot of oil going on and you definitely want to do a second shampoo. And if you pay attention on the second shampoo, you'll probably only need to use half as much, so you'll save a lot of money and it'll be easier to rinse that product out. It's so important to rinse the product all the way out or else you are going to get flaking and you might even get hair that's weighed down."

When it comes to conditioner, Kyan says, "You definitely want to put enough product in so it's covering your hair, but if you're really rinsing out your shampoo completely then that conditioner is going to go right in there and go to work. Conditioner is meant to stay so you don't want to use too much or else your hair will get weighed down."

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