1. Navigation (compass and maps in a watertight case)
2. Sun Protection
3. Insulation (extra clothing)
4. Illumination (headlamp or flashlight with extra batteries)
5. First-aid kit
6. Fire (water-proof matches or butane lighter)
7. Repair kit and tools
8. Nutrition (no-cook foods)
9. Hydration (including filters)
10. Emergency shelter
For me the question is when to take all this stuff along. Ideally you’d never leave the trailhead without the whole assortment, but it seems like overkill if it’s a short walk on trails you know by heart.
My rule is: if it’s over 10 miles on unfamiliar trails, I take the Essentials. Since I own an ultralight tarp and bivy, I often throw them in my daypack. The bonus is getting some “use” out of something I paid good money for, though with any luck they’ll never leave their stuff sacks.
And here’s an extra special lost-hiker story from USA Today to help all the points sink in.