Know about your destination. How do people dress? Will you have access to laundry facilities?
For instance, I travel to Paris every couple of years. I know that, except for the people in my party, noone will notice or care if I wore "that shirt" yesterday!
Make sure that every shirt goes with every pair of pants and/or skirt. If you can wear jeans, you only need 2 pair for a week or more.
Another suggestion I have is to pack everything, and then haul your luggage around your neighborhood and up and down your stairs for a half an hour. If you are over packed, you will know it after that exersize!