You are half way to perfect hair when you stop to damage it the way most of us constantly do: bad combs, brushing without detangling, rough towel-drying, and snaggy hair clips. Now, hair is at its most vulnerable when it´s wet (that´s because water breaks the hair bonds). That is why any kind of rubbing while washing your hair is very very bad for it.
The cup method is an easy way to prevent the rubbing while lathering the shampoo on our hair. You pre-lather the shampoo in a cup. To do that you need a cup with a lid, or a jar, or even an empty bottle. You pour in a small quantity of shampoo (or conditioner, if you CO), add a bit of water, close the lid and mix well. The proportions of water to shampoo / conditioner are up to you, but it should be rather runny. You then apply the mixture evenly all over your scalp, you will notice that this is much easier to do it when it´s runnier. Now you only need to gently massage the scalp and rinse out.
Update: You can also use an empty bottle instead of a cup, just shake to lather!
You might also want to read my post on washing the hair the right way.