Here's some tips to follow before you hit the trails.
1.) Check your tack. Make sure that all tack is in proper working condition. All leather is not frayed or broken or about to break ; bridles, reins, girth strap. It is also good to use a breast collar, especially if the trail you'll be on is rugged and hilly. It will help to keep your saddle from slipping back.
2.) If the trail you'll be going on is unfamiliar, take a practice run without your horse and obtain maps and brochures about the trail. Check out the trail thoroughly so that there are no unexpected surprises you and your horse may not be familiar with.
3.) Don't go it alone. It's best that you plan ahead and bring a trustworthy friend and provide that person with a map as well. Don't rely on only one person carrying the map in case something was to happen to the person carrying the map. Don't put all your eggs in one basket.
4.) Bring a cell phone, only carry it on your person not in a saddle bag. If you and your horse have departed each other, the cell phone won't do you any good if it is traveling down the trail with your horse and you are not. I know some smart horses, but none that can use a phone, well, except for Mr. Ed.
5.) If it's going to be a long day, make sure you bring some hay for the horses and some water or buckets for water. One of the trails I go to does provide water.
6.) A first aid kit for you and for the horses. Include a hoof pick, wraps for leg injuries, temporary rubber boot/shoes for shod horses that may lose a shoe on the trail.
7.) Condition your horse before you hit the trail. On those first trail rides, keep them short and less strenuous. Let your horse build up to the long and hilly trails.
8.) Always let some one know where you'll be and approximately how long you'll be.
9.) Respect the trail system that you are on and follow the rules.
10.) And before you load the horses, make sure that all is working on your trailer and truck/car. Check that brakes and lights are in working order and you have a full tank of gas. Be prepared.
11.) Make sure your horses are up to date on vaccinations and current coggins and your paperwork is in order.
If anyone else has any suggestions that I may have missed, please feel free to comment.