The Five Essentials

I'm sure you've heard of them, or have your own stash. 

The Ten Essentials.

Now refined to the Ten Essential systems many backpackers, hikers, hunters, and outdoor enthusiasts of all types have come to respect this list of wise choices for the backcountry. Many lives have no doubt been saved by these precious few simple items. 

If you are not familiar with them they are:

1. Navigation
2. Sun Protection
3. Insulation
4. Illumination
5. Fire
6. Repair
7. Nutrition
8. Hydration
9. First Aid
10. Shelter

But I when I poll my friends and fellow adventures I find it hard to find someone who regularly carries all ten. Many of them just as a matter of course take items because they are necessary, ignoring the list. 

Why? Between different activities, styles, and choices, and climates there is a need for different tools. Different days require different items, and the list is still to rigid to accommodate those changes. 

But I think the other problem is the list is very gear centric. I've seen it on forums, peaple comparing "load outs" and which item is best. Consumerism is in inherent part of the outdoors, for many people the experience of new gear is their draw, and that's okay. But some of us also need to move away from looking at the outdoors purely the lens of what tools we need. If you ask me the safest system isn't based in fancy gizmos, it's foundation is a solid plan. It's not material, but it's the most important part of your list, and your best safety mechanism.

Basically, don't get too caught up. Start slow, make mistakes and learn what works for you.

In light of that here is my amended FIVE SYSTEMS OF DAY HIKING (with backpacking added below)

1. The Plan. The vast majority of accidents happen because people often have not researched their route ahead of time, or have deviated from what they know. Have a route planned out, friends to go with and a plan, leave it with a trustworthy individual and go from there. The plan should include up to date weather information, a chosen route, info from a guidebook regarding difficulty, and nature of the route as well as reliable navigation means if the trail becomes difficult to follow. Your iPhone and it's mediocre battery life only counts if you've got backup batteries). Personally I have had a lot of scary experiences relying heavily GPS. The Old Map and compass method, with GPS as an assistance rather then a cornerstone is my preferred method. I never use more than UTMI and a topographical map. 
2. Sustenance - Where you live determines how much is required and of what. A very rough rule of thumb is 1 litre of water for every 4 hours. For every 5 degrees Celsius over 15C add 0.25 litres of water. Or bring purification pumps or tablets if your skittish about the naturally flowing water in your region. Hydration bladders mean you drink more and stay better hydrated, and you move faster as you don't have to stop and drink. Water bottles force you to ration your water, as you are always aware of how much is left, and you have to stop each time to drink. For food bring I prefer not eat full meals during the day. Instead I have bars, gels, meat sticks and snack through the day. Try to only use less-processed foods that release energy slower. A mix of carbohydrates, fats and proteins is ideal. This is why Trail Mix is ideal.
3. Weather Protection - This depends on where you live. Generally though you need protection from: The Sun, Wind, Water, and Cold. If you started with The Plan, then you should know what you will need. If you don't then best to find Sunscreen, hat, sunglasses, rain jacket, insulation, are generally good to have no matter where you are in the summer. When you are starting out don't guess when it comes to clothes, over pack and experiment. As you become familiar with your climate make educated guesses. Always dress in layers, never cotton. Quick drying pants and a PU rain jacket are inexpensive and the best places to start. If there is wet conditions put in some sealable plastic bags for paper and electronics. A Garbage bag liner for your pack is also very functional. Upgrade to drybags later on. Footwear is another important type of protection. Good torsionally rigid footwear is key for backpacking.
4. Extras - Extra insulating layer, extra socks, extra batteries, extra snacks and some money if you are near civilization. These are all handy things to have just a bit more of. It's definitely handy to have a backup, or a just in case layer for the unforeseen. I consider trekking poles a essential extra. They have tons of uses. A Sil-tarp is another handy piece of kit.
5. Sh*t Hit the Fan - Your plan didn't work out. First Aid Kit, Duct Tape, Headlamp are all items you probably didn't need until now. If you are in an area where exposure is serious issue (cold climates, rapidly changing weather or temperatures erratic weather etc.) bring a some form of shelter. A good quality Reflective Space Blanket for $10 makes for great warmth, a tarp, and is super lightweight.

For Backpacking there are naturally a few more things that get added on here

1. Night Stuff (Shelter) - At it's most basic this could be just a tarp or bivy bag with a mat. Generally though a lightweight tent, sleeping mat(you loose most of your heat through the ground so never leave your mat), sleeping bag, and a headlamp are ideal.
2. Meals - While everyone is different plan for 3,000 calories per day minimum. For Dinners, I prefer dehydrated meals to save weight. They can be slow to cook, but they are definitely tastier than freeze dried. Planning ahead and trying to make ingredients (like tortilla shells, bagels, powdered eggs, powdered milk) shared into other meals can reduce weight. These websites are handy for all budgets: MSR Recipes Archives, alpine cooking , The Best Camping Food . For breakfast I like coffee, and something that is quick and easy, like oatmeal, bars, or bagels. I am terribly lazy in the morning and the sooner I am on the trail the better.
3. Comfort Items - These aren't essential but worth thinking about. Things like camping pillow, a book to read, an iPod (personally I don't like to much tech in the backcountry as for me it ruins the aesthetic, but that is a totally personal preference).

Four Piles

Like anything this system should be something to develop. The best way to do so is to use a ultralight packing technique you've no doubt heard of.

 Every time you come back from a trip, unpack and make four piles. In one put the items that you used lot's or couldn't have had fun without (i.e sleeping bag, shoes, etc.). In the second pile put the items you used a bit. In the third pile put the things you didn't use at all. In the fourth put what you wish you had brought on the trip. It is a handy way to loose weight, or specify what makes a good backpacking experience.

There are plenty more cool gizmos and gadgets, but in terms of what one needs, I think this covers it.