There are as many ways to hike the Appalachian Trail as there are switchbacks. Still, here are a few dollops of wisdom that should apply to most would-be thru-hikers.
Choose the right backpack
Fit is everything. The proper pack makes a 40-pound load feel like 30, while an ill-fitting or flimsy pack makes that same weight feel like a bag of bricks.
Choose a full-size internal or external pack. Anything smaller (3,500 cubic inches or less) is rarely equipped to carry heavy loads in comfort. Above all, find an expert (an experienced friend and/or store associate) to adjust the pack to your body.
Add variety to the AT hike menu
It may not have seemed like a big deal as you stuffed the same contents into each of your mail-drop packages* prior to the hike, but just wait until your 50th straight day of mac & cheese. At that point, you'll be avoiding the post office altogether. Mix things up a little and never repeat a meal in the same a week.
*A mail drop is a package of supplies sent to a post office prior to your arrival.
Pack some "happy" food
It's going to happen during your hike. Your morale will hit zero and you will find yourself stuck at the wrong end of a long upgrade, totally devoid of motivation. That's when you break out the candy bar (or something equivalently tasty and bad for you). It may not be nutritious, but it will get you up the mountain.
Don't scrimp on the hiking calories
The typical thru-hiker consumes 4,000 to 5,000 calories each day (and still wants more). When browsing the grocery stores, pick food that is high in calories and carbs. Just think, this might be the only time in your adult life when you can eat as much as you want and still be guaranteed to lose weight.
Leave the dictionary at home
I figured to bone up on my vocabulary during the six-month hike by bringing along a dictionary. You can imagine the results. The motto is: Minimize weight whenever possible. Your back and shoulders will thank you.
Before starting your hike, have someone more experienced pick out what is too heavy or just unnecessary. Like that eight pack of spare D-cells or the 400-page guide to North American birds.
Oddly, the dictionary thing kind of worked out. I was caught in a spring snowstorm and had to stay in a lean-to for an entire day. To warm my hands, I started burning the book one page at a time. I got through the K's before deciding to save the rest of the alphabet for another nor' easter.
Find a backpacker's poncho that is extra long in the back (to cover the pack). A poncho has great ventilation and protects your torso and thighs. It's much lighter (and cheaper) than a combination of rain jacket, rain pants, and pack cover
Bring extra socks
During my hike I brought along six pairs and rotated them constantly to ensure that the pair on my feet was always clean and dry. Except for one in the first week, I never experienced another blister from Maine to Georgia.
Backpacking means bugs
Bring a hat, a long-sleeve shirt, a head net, and bug lotion. Although there's a great lean-to system on the A.T., you should still bring a tent. You'll be thankful for an enclosed shelter on those nights when the mosquitoes are so thick they're jostling for air space.
Test your gear first
Day 1 on the A.T. should not be the first time you wear your backpack with a load, fire up that shiny new stove, or break-in thick leather boots. Take it all out on a trial run first. Spend a few days and nights hiking in the woods to ensure that your 20-ounce sleeping bag will keep you warm enough, your pack fits, and your stomach can actually handle all that freeze-dried food.
Know what you're getting into
Hiking the A.T. is beautiful, inspirational, and life-changing. It is also very, very hard. You are most always hungry, grimy, and in some degree of pain. A typical day consists of 12 hours of hiking up steep mountains with a 40-pound pack followed by 12 hours in a coma. Many prospective thru-hikers fail to finish because they underestimate the hardships. Give it some forethought, and make sure that you are just crazy enough to enjoy the A.T. lifestyle.