No jeans, cotton undies or T-shirts, period. Eat your fattiest foods first. Cheeses, eggy stuff and the like spoil fast. Especially at the very start of a trip. As you are driving to the trailhead, start pounding water down.
Veterans can load up nearly a gallon before starting the hike.
Don't use a flashlight at night. If there is anywhere near enough moonlight, you will probably be able to see better without a flashlight than with one. You might also consider fastening a red, transparent filter over your flashlight. Astronomers and stargazers use these when reading star maps. Have a trip motto.