No fancy verbiage here. No disclaimers or qualifiers. The following article will show you how to pack. Pack for a day trip, pack for moving your entire collection of earthly possessions, pack for hiking... it's all here.
This article is not just for military members, in spite of some of the techniques, it's for anybody that wants to learn how to travel or move items efficiently, hikers, campers, etc.
...bags you would like to take.
If you're hiking, obviously, for the majority of cases, you are going to be taking one, which fits on your back, and needs to be loaded at least marginally optimally to put the weight a little higher up on your back to make the hike as easy as possible.
If you're traveling on a plane, you obviously are limited to a single checked bag and a small (and I do mean small! Don't be the asshole trying to use Astroglide and his feet to jam a bag into the overhead bin, or taking up all of your leg room with some huge item) piece of luggage such as a backpack or gym bag.
If you're moving, however many boxes you can buy/scrounge up and fit in your car or truck.
Start off by deducing this parameter from the items required, which leads us right into:
...do you need to take along?
If you're not moving or going on an extended journey, start by thinking about what you're going to need. Then, divide that number by half.
If you're going on a vacation, you're going to be spending money anyway. Why not divert a little of that towards buying things like toiletries, entertainment, and extra fluffy pillows when you get to your destination?
Take advantage of "travel-sized" items. Instead of trying to lug your laptop along, bring a book. Instead of bringing a 5-lb. bag of almonds to snack on, put some in a ziplock bag, etc., etc.
A good step here is to lay everything you're planning to take out on your bed/another flat surface before even starting to pack. Take a good, long look, and then start to put things back where you got them. Do you need a pair of sneakers and running shoes? Do you really need jeans in Florida? These are all questions which can help you cut back on weight and space.
Look at all that extra space!
Now that you have everything laid out, start packing.
Start by converting your bag into the world's largest, most permeable condom. Roll the sides down until you have a little less than a foot of total height. This will help you really pack the bottom of your bag tightly.
Now, start with oddly shaped items and hollow items (like shoes) first, while you still have plenty of "filler" to use, like socks and T-shirts.
Fill your shoes with any small, non-crush-able items you may have. If any of these happen to contain liquids, put them in a ziplock bag unless you want to be running around in shoes that smell like Chanel.
If you have aerosol cans, make sure they have a cap, and then tape that sucker on (electrical tape works well and doesn't leave behind the same residue as duct tape).
After your shoes are filled, "cap" them off with some socks. You can then band shoes together with soles outward, heels at opposite sides (like they come in the box new), with either rubber bands or tape.
Place these, along with any other durable oddly-shaped items, at the bottom of the back. You can optionally make a layer under these by folding flat things like extra (empty) luggage, cardboard, etc.
Now, pack the area surrounding these items using rolled t-shirts, socks, and other items of clothing/bedding/fabric, or flexible (durable) items.
Work one layer at a time. All of your clothes should be rolled, not folded, this:
Pack each layer like you would stack a pallet of bricks. If you've never done this (a majority), make one half of the layer with rolls of clothing aligned "north-south", and the other half "east-west", and alternate between layers.
It's here you should start thinking about what you need at the top. Clothes that need to be relatively wrinkle-free on arrival, and fragile items should obviously go at the top of the bag.
If you're not packing one bag, especially on an airplane, think about what you need in your carry-on. Things that absolutely can't get lost in baggage claim are a good idea. For military types, a complete uniform (for whatever day and destination) is a good idea (unless you're travelling in it... boot).
Several products can make this a lot easier and save on even more space:
What you should take is up to you. I won't go preaching here because each trip is different, and what you may or may not need is highly variable.
I also won't say what should be more easily accessible than the other. Use your own brain, and the tips above to plan accordingly.
I will, however, talk about weight distribution.
With hiking, start by putting lighter stuff (like clothing) towards the bottom of your pack.
Heavier items should be placed more towards the top with whatever means you can contrive to keep them crushing what's below them. Most packs are compartmentalized for this purpose. If not, straps can be used to "hang" things near your shoulders.
Water, gear, etc. should be placed above softer items to bring the center of gravity of the pack near your shoulders. A little trial-and-error can do this quite nicely.
This article was largely written from my perspective as a Marine who doesn't like to lug more than a couple of bags through a travel terminal.
At the end of the day, would you rather be carrying two large suitcases, your carry-on, and chasing your kid through the airport, or would you rather take a single suitcase and some kind of messenger bag carry-on, keep control of your rugrat, and do without a few stupid things you wouldn't have needed anyway, along with spending a little extra time on packing?
The answer should be obvious, but I leave it up to you. :D