I'm an Eagle Scout, and the boy scout motto drilled into all of our heads was always "Be Prepared". I made the mistake of being OVER prepared for years when I was a kid; multiple flashlights, extra batteries, more food than I would eat, a ton of water. It took some learning, but a few no-brainer gear cuts that I now make all the time:
1. Extra batteries. Unless you're going to be camping for more than a month, make sure the ones you have are good and ditch the extras.
2. Extra water. The single heaviest thing you'll carry. It's easier to carry enough water for the trail, and boil water clean when you make camp.