First of all, know the dangers of overwashing. Most people should not be shampooing their hair every single day. Overwashing is bad for numerous reasons, one is that it strips your hair of its natural oils, and these are critical in protecting your hair from damage. It also kicks your oil glands into high gear as they try to keep up. People who overwash often claim their roots get greasy very fast, and don't realize that their overwashing is the reason why. Lastly, overwashing causes your hair to get dry ends.
Shampoo should have a ph level of 4.5 - 6 because it will close the cuticles of most healthy hair (which is ideal for holding in moisture and keeping out the bad stuff). Most professional brands of shampoo will have a good ph level.
Clarify or Die
You'll also want to choose a clarifying shampoo if and when you need it. You'll know if you need it if your hair is dull, hard to style, flat and possibly flaking. Clarifying shampoo will remove buildups that can prevent active ingredient products from being effective.
Make sure to shampoo thoroughly! If you have thick hair it's a good idea to section it to make sure to get it all. Scrub in circular motions, focusing on the roots. Most of the time, you do not need to shampoo the ends. Some product lines (like Davines) recommend shampooing twice.
Always wash/rinse your hair in cool water if you want to preserve your colour, or if you have very curly hair that is prone to frizz. Wash your hair with warm water if you have particularly dirty hair, as the heat helps to break down and remove debris and buildup.
Conditioning is not an option
Be generous with conditioner on your midshaft and on your ends. Only work it into the roots if you think your scalp could benefit from it; for example if your scalp is dry and your conditioner has moisturizing oils in it that skin can absorb. Let the conditioner sit for 2-10 minutes. If you have very curly hair or tough mats, during this time you may want to comb it out with a wide tooth comb. Rinse the conditioner with cool water to seal in moisture and shine. This is because the cool water closes the cuticle. Again, this also prevents frizz.
Dry them tresses
For curly hair or over-processed hair it is important to reduce friction when drying it as much as possible. Instead of scrubbing it with a towel, pat it dry or wrap it up in a cotton tee.
Call it a science or an art, but there definitely is technique to washing hair, and I hope you learned at least one thing reading this. Happy sudsing, all!