I encourage my friends to dress entirely in quick-drying synthetic clothing, such as nylon, polyester, and fleece. Quick-drying synthetics protect you against hypothermia. I mean every article of clothing! If you're skeptical, type "cotton kills" into your favorite search engine and ponder. Many cases of hypothermia occur when it's in the 40's or 50's F.
I also very much like everyone in the group to wear a whistle. It is easy to become disoriented on the trail, or after stepping off the trail. Three toots on your whistle will help your partners find you. The blast of the whistle carries much farther than a shout, and is much less fatiguing to repeat. The right kind of whistle is called a signal whistle or survival whistle. Plastic conducts less body heat and won't feel chilly.
The Mountaineers, the Pacific Northwest hiking club, began teaching members about safety gear
in the 1930's. Their list is stilled called the "Ten Essentials":
1. Topographic map (Know how to read it!)
2. Compass (ditto, including correcting for magnetic declination)
3. Flashlight or headlamp
4. Extra food
5. Extra clothes
6. Sunglasses--real UV blockers!
7. First Aid kit (Consider taking a class in Wilderness First Aid.)
9. Waterproof matches
10. Firestarter material (such as cotton balls soaked in petroleum jelly)
Most important item: take your brain, on the setting: "calm and creative"!
(c)Rhona Mahony, 2002: firstname.lastname@example.org