Not how often, but how?
It’s better to shampoo your hair twice, not once. This is because the surface (the cuticle) of each hair is scaled, like a fish. When you shampoo, this lifts, or opens up, the scales – that’s why your hair feels ‘rough’ or coarse after shampooing. But it’s actually the second shampoo that gets in and really cleans the inside of the hair. Conditioning then
seals the hair; the scales are now closed and the hair feels smooth again.
So if you’re going to shampoo your hair, you’re far better off doing two small back-to-back half shampoos than one big one. Try it – you’ll feel the difference immediately, and you needn’t use any more shampoo than you normally would.
And please condition. Your hair may feel thicker if you don’t, but your hair will be ‘open’ still, filling with dirt, which defeats the purpose of shampooing in the first place. If you need volume, use a styling product or ask your stylist for professional advice.