Shampoo has existed for a really long time and there are many different kinds available today, from mass produced, jumbo sized synthetic cheapo bottles to its exact opposite.
Regardless of its many varieties, I know but 4 standard methods of head cleaning.
1. Shampoo only - usually done by those with uber short hair, this is the quickest way to clean your head. I've used this method when my hair is short, I'm feeling too lazy to condition or I've done a deep conditioning mask a day before hand.
2. Shampoo then conditioner - the most common way a girl washes her hair, as inundated by a ton of beauty products and ads. Unfortunately, it also takes way too long to clean my hair sine I'd have to do do a rinse for both shampoo and conditioner. I reserve this method for deeper hair cleaning sessions and dirtier surroundings such as after extensive labor.
3. Conditioner then shampoo (Reverse 'Poo) - not quite as common as the ones above and below it, I prefer this method since it cuts my shower time by at least 20%.
4. Condioner only (No 'Poo) - gained in popularity a couple of years ago, but not too much in hot, humid and polluted surroundings like Manila. I've tried this method a couple of years ago when I was on break and had to give up on Day 5, even without a heavy workload. It just doesn't feel as good or clean as using something with shampoo.
Which way is best?
Each one has its own benefits and drawbacks. For myself, I prefer method number 3 because it cuts both the time I spend in the shower (always important for the early morning rush) and the amount of water hair care takes per day: I usually need just one rinse for method 3 and at least two for method 2. In the long run, I think I'm saving more water, a scarce resource- which is a plus.
Moreover, I recommend trying different ways to wash your hair in a way that suits your hair, your lifestyle and your beliefs. Who knows, you may find something which works better for you.