1. Make sure someone knows where you are going. Have a contact person to call when you get off of the trail.
2. Always carry plenty of water. Carry more then you think you are going to need.
3. Have a sturdy day pack to carry some emergency supplies.
4. Supplies: an emergency kit with a mirror, space blanket and some general first aid supplies, such as bandaids. I always carry duck tape wrapped around my water bottle. Duct tape has fixed many of my hiking gear. Flashlight is a must. Snacks. I always carry some form of fire starter, just to be on the safe side.
5. Do your research about the weather. What kind of temperatures? Change of socks is always good. If it is going to be cold, take layers to put on or take off according to your activity. It is extremely dangerous to get caught in cooler conditions in cotton. You do not want to have soaking wet clothing and catch a chill. Cotton does not dry fast enough to keep you warm.
6. Now, the important information. Always keep in mind how long you have been on the trail, when the sun goes down and how long it is going to take you to get out. If you are prepared with a flashlight and the correct gear to get you thru the night, it is not going to matter. If you are day hiking and you do not have the correct gear, turn around in time to give you plenty of time to get back to the trail head. Do not be concerned with reaching the top. The peak is not going any where and you can hike it later when you are better prepared.
As I stated before, these are all suggestions. We have witnessed many different emergencies on the mountain and a lot of them could have been avoided if they followed some of these suggestions. I hope these help with your next day hike. Happy Hiking.