Several organizations and wilderness experts have published lists of essential items that need to go into your Day Pack for trips into the wilderness.  After reviewing a large number of them and applying our own personal experience, we condensed them into the following list.

For short hikes or day hikes you can use your judgement regarding some of the items - but do remember to factor in the season/climate, terrain and the distance you are planning to cover.

1. Map of the area (in waterproof clear plastic bag)
2. Navigation Aid (Compass / GPS receiver)
3. Insulation (Extra clothing / layers / raingear / socks / gloves )
4. Illumination (Flashlight / headlight - LEDs recommended)
5. Extra food
6. Extra water & water purification tablets
7. Sun Protection (Hat, sunglasses and sunscreen)
8. Pocket Knife/Multi-purpose Tools
9. Fire (starter and fuel)
10. First aid kit

Some additional items -

• Toilet paper (carry in plastic bag)
• Plastic Bags
• Small shovel
• Insect repellant
• Signaling device (whistle)
• Prescription glasses (an extra pair)

Remember to check your gear/supplies before each trip (e.g. batteries for your flashlight) and become familiar with the tools and techniques (e.g. how to start a fire or purify water) before you face that situation in the wilderness!

It's always a good idea to hike with a companion - unless you are very familiar with the trail and you know for sure that other hikers use that trail at least a few times a day.  

Additional Reading:  We like the wording/categories of the "The New Ten Essentials" [ Details Here ] from the Sierra Club though for the sake of completeness, here is a link to the original "The Ten Essentials for Backcountry Travel" [ Details Here ].