nichole is right....washing cleans off dirt, but over washing removes the necessary natural oils from your hair. So don't shampoo twice (like the bottle says), don't wash too long or with water that's too hot either.
It depends on the person. Some people wash there hair everyday, some once a week, and some twice a month. My African American female friends don't like to get there hair wet because it makes it "poof". Though that's not true for for all African Americans.
As least often as possible, if you can, use a dry shampoo for a extra day.
One should shampoo twice, first one lifts the dirt and the second removes any residual dirt or product. You should only have to use half the shampoo on the second wash or less. Apply conditioner away from the scalp.