The biggest myth is that washing your hair frequently is bad, but there is no real rationale for that. You take your hair everywhere you go; it picks up dirt and pollution, and you’ve got sweat, oil and dead skin cells.
You can wash it every day if you want to. You are not stripping the natural oils from your hair, because they do not travel from the scalp down the hair shaft, especially if your hair is damaged or colour-processed. Physiologically, all hair is the same. The difference is in the shape of the strands.
Although there has been a trend for washing only with conditioner, generally you do need to use shampoos. They contain a surfactant, which is designed to hold in dirt and oils that can then be rinsed from the hair.
Get your hair thoroughly wet. If you wash your hair twice and the shampoo seems to lather better on the second time, it’s because your hair is properly wet. A 50p-sized amount of shampoo is enough for most people. Wash the scalp rather than the hair and allow the suds to run down.