Keep your maps in a plastic bag to protect them from the rain and know how to use them. GPS units are great, but they are not substitutes for knowing how to use a map and compass.
Hopefully you won’t ever need to use it, but what happens if the batteries in your GPS are dead?
It protects the environment and prevents you from getting lost.
Weather can change quickly in the mountains. Always tuck rain gear into your backpack and bring along layers of clothes.
Let someone know where you will be and when you plan to return. On a long-distance hike, leave behind a copy of your itinerary.
Make sure you have the supplies to deal with major injuries, and make sure you have the knowledge to use it.