"The Ten Essentials" were first identified by The Mountaineers in 1930. They were updated in the early 2000s and now include Ten Essential "Systems":
1. Navigation (map and compass)
2. Sun protection (sunglasses and sunscreen)
3. Insulation (extra clothing)
4. Illumination (headlamp/flashlight)
5. First-aid supplies
6. Fire (waterproof matches/lighter/candles)
7. Repair kit and tools
8. Nutrition (extra food)
9. Hydration (extra water)
10. Emergency shelter
Dressing for the conditions you expect to encounter are easier now than ever. Explorers used to dress in wool to stay warm. Now dressing in layers makes everyone a bit more comfortable. For cold weather, general recommendations include a moisture wicking garment (polypropylene or a cotton-polyester mix) next to the skin and a wind and water proof outer layer, such as Gore-Tex. For warmth, fleeces can be used, preferably something with armpit zippers to allow sweat to evaporate when active.
For summer months, the moisture wicking element is critical during long hikes. Conventional cotton clothing can hold moisture against the skin and decrease evaporation and cooling in humid environments, resulting in heat exhaustion.
Bottom line, there is no longer a need for straight cotton shirts.