In Your Pack:
Some items for the trail
Water - Even on a cool day and a short hike, do not leave your vehicle without it.
GPS - Always have fresh batteries in it and spares in your pack. Even though the GPS is great, you still need to be aware of your surroundings, where you are heading, and where you have been. Turn around and look behind you so that you recognize the area when you are returning - it will look different going the other way. It is always a safe bet to mark your trail with a stack of rocks or scratches in the dirt pointing to the way out. This may seem redundant, but you just never know... and yes when using your GPS, do not forget to "mark" the position of your vehicle or where you started the hike.
Maps - Physical maps of the area you are hiking.
Sunscreen & Chap Stick - No scented sunscreens. The scents are really nice next to the pool, but they can also attract an assortment of unwanted flying critters. Chap Stick or a lip balm with a sunscreen in the formula is helpful for extra protection.
Gators - They are normally used to keep snow out of your boots. Low-cut lightweight ones help keep weeds out of socks and boots.
Hat and jacket - Protects you from both heat/sun and cold.
Medication - You may be out longer than expected or the exertion may require you to take extra meds.
Sunglasses - Pretty obvious...
Two-way radios - FRS radios are great for keeping in contact with your hiking partner. Make sure you have new batteries every time you go out and that you are both on the same channel. I have hiked with a person who did not carry extra batteries and did not change the batteries in his equipment and it could have been really dangerous if we had needed to get in touch with each other. The same applies for your GPS; keep new batteries in it and fresh spares in your pack.
Cell phone - Keep in mind most cell phones DO NOT work in back country. Know your coverage areas and do not assume that your phone will work. Now if you are fortunate enough to own a Satellite phone, please keep me in mind the next time you go out.
Flashlights and fresh spare batteries
First aid kit, tweezers, insect repellent
Gloves - I know, real men don't eat quiche or wear gloves. Those of us who have hiked the washes full of cactus or have hiked the lava fields that can be sharp as razors, may beg to differ.
Walking stick - Many people do not like them but for me it has have been a virtual lifesaver. Not only is it a "third leg" when coming down steep slopes, it also helps this old body up the steep slopes. I also use it for beating the bushes / making noise when walking in areas that may be hazardous to my health.
Camera - Do not leave home without a digital camera.
Paper and pencil - Handy when leaving a note on your vehicle for someone to find.
Hi-energy food bars - Just in case you are out longer than expected.
Hike with at least one other person - Many of us do hike alone at times and that is not a really good idea.
Know Your Limitations - Even though this is listed last, it really is one of the most important points.