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It gets very cold in the mountain at night. I stuffed my sleeping bag with the clothes I would wear the next day so that they would be warm when I changed. It also added an extra layer of fabric at my feet when I slept.
Do not carry extra liters of water. You may be tempted to for fear of running out of eater, but they are plenty of places to re-fill and buy water. Carrying excess weight will make your ascent and descent more tiresome.
Use a water bladder. I wish I had this, but I opted for a Klean Kanteen and Nalgene bottle. Having a bladder just makes drinking easier. You don't have to stop in the middle of the hike to grab your water. Because it is also very handy, you can drink as often as you would like and it makes hydration so much easier.
Reverse or remove the batteries on your headlamps during the day so that they don't accidentally run it out. The headlamp is a vital piece of gear at night especially when going to the bathroom or when trekking early in the morning. You want to make sure the batteries still have juice in them to the last day.
Energy bars really do help. Stock up on those energy chews and keep them handy. Sugar sometimes tastes better on the high altitude than jerky.
Keep your pack as light as possible. Even though a portion of your things will be carried by porters, you'll most likely still carry a day pack. Keep the items in here to the essentials.
Pack only what you need. You'll be waking up early in the mornings, in the cold and in the dark. You don't want to be re-packing your things each day since it takes time and makes you more tired in the high altitude. I made a mistake of bringing a book. Although, it didn't weigh that much, it still was a few ounces. I could have done without it for a few days especially since I was too tired to sit down and read.
Trekking Poles are a lifesaver. Don't forget to bring or rent them. They will save your knees and provide you an extra set of footing as you climb and go down narrow and uneven steps. Some parts of the Inca Trail are very narrow. Having trekking poles allow you to get a feel for the steps before your feet land on the ground. Make sure you adjust them accordingly: Longer if descending, Elbow height if ascending.
I wore thin merino wool long sleeves during the day over a dry-wicking tank top. I got hot, but the wool dried quickly enough. Long sleeves also prevented me from needing to put on too much sunscreen or bug spray which meant less chemicals I had to wipe off of my body at the end of the day. The long sleeves saved me from also getting sun burned in the high altitude. Even though, it is cool in the mountains, you are higher up and can get sun burned really quickly and easily.
Don't hold your pee. One of the girls in our group may have gotten a UTI from holding it. Don't wait to use the bathroom until one is available. There are plenty of trees and high brush to cover you while peeing. You don't want to spend 2-3 more days being uncomfortable in the trek with a UTI.