Back in undergrad (two years ago now), I took an outdoor skills class. I enjoy doing stuff outside, and I figured it was a good way to learn more about camping.
In the class, our instructor, Chad, was big on little things that can make a big difference, survival wise. We talked a lot about carrying items that had more than one use, and he insisted that we carry all quick-dry clothing (cotton retains water and can be dangerous if it gets cold and rainy). He had a lot of funny, quick slogans, like "Cotton kills!" and "If you're not peeing, you're not having fun," (to ward off dehydration).
One favorite phrase was "see me, save me!" The premise is that a piece or two of brightly colored clothing could save your life while hiking. For starters, quick identification as a human could keep you from being shot if you happen to hike through an open hunting area. Second, being easily identifiable can be a big assistance to search and rescue crews if you're ever lost.
If you incorporate your "see me, save me" item into your wardrobe, you get double duty: you've got your piece of clothing, and it also comes in handy for life savings.
I carry a red bandanna with me when I go hiking as one piece, but my favorite see me, save me clothing is my green fleece jacket. While it has faded from the initial very bright "Margarita" green it was when I purchased it, I still think it serves its purpose:
Photo taken in Glacier Nat'l Park last summer as I was moving cross-country. This jacket can be seen in a huge chunk of my traveling/camping photos.