Common Sense - you're well read and have spent time with other experienced people in the outdoors - read, read, and read some more on what you're going to do - trail guides, technical guides, gear guides, natural history guides, weather guides, medical guides, maps, newspaper and magazine articles. See my resource area for books to get started.
Proper clothing and extra clothing - layers - extra fleece - wind and rain shell - hat - extra socks - synthetics only, no cotton
Food and Water - more than you think you need - water filter if on longer trip
First Aid Kit - always carry - ibuprofen - moleskin - bandaids - antibiotic cream - insect bite antiseptic - gauze - small scissors - tweezers - straight pin - space blanket - ace bandage
Map of where you're going, that you've studied and know how to read
Compass or GPS - know how to use what ever you choose (if GPS, then extra batteries)