Before you start assembling the items for your 72 Hour Kit, define what survive means to you. If you come out alive, regardless of missing a limb due to frostbite, is one end of the spectrum while a spare Winnebago might be the other end. I generally fall in the middle- I don't mind suffering a bit but would rather keep my digits.
Another decision you need to make early is whether you want a ‘one time use kit' with disposable items, or a kit that can be reused. This becomes quite important for two reasons, the first being that a one time use kit will only get used when death is on the line, the second is that the ‘one time use' kits fall into the “barely survive- maybe even without my fingers or toes” end of the spectrum. A reusable kit can become part of a lifestyle, turning many “disasters” into inconveniences or even adventures.
One last item- DO NOT OVERPACK. It is tempting and it will take some discipline, resist the urge to say “I might need this”. A kit that is too large or too heavy is many times just as useless as no kit at all.
Shelter. This is the highest priority. The sun will bake you, the rain will freeze you (hypothermia can set in quickly during the fall, spring, and winter if you are wet), and unless you are lucky enough to be stuck in a benign environment (like Tahiti), lack of shelter will be more than an inconvenience. Shelter can be a tent, tarp, or a bivy sack (I don't consider the ubiquitous tube tent an option). The best thing I have found is a rip stop nylon poncho that folds flat and can be used as a tent. These are very versatile, they are lightweight, very tough, they do double or triple duty (a must if your kit is to be light enough to carry), and some of these can be used together to make a larger tent if you are with others. Make sure you have a small rope, stakes and a large, strong plastic trash bag (to slide your bedding into) along.
Water. There are recommendations galore for how much water to put into your kit. My opinion is 30-60 oz per day, depending on your size. If you are very small take the former, larger the latter. If you try to get by with less than this you will start to gamble with your life on many occasions. I have to note that the government says one gallon per person per day- but this includes water for washing, cooking, etc, all of which I deal with later.
Options for water include sterilized boxes, sterilized mylar bags, water bottles, and reusable containers. The sterilized water (bag or box) are awfully convenient if you don't let their relative inavailability keep you from using them when you need to. They have a 5 yr shelf life and you can just forget about them. Water bottles have to be replaced yearly and take up a lot of extra space due to their shape- but are otherwise good. You can also get reusable containers (generally collapsible plastic) which work well if you sterilize the water (and keep it sterilized) or wash them and replace the water often. I don't consider water purification tablets an option due to their taste and the fact that water isn't available in any form during some crisis.
Food. Take 2,000 calories per person per day. There are plenty of options but the best require no preparation, no utensils, and no dishes to eat. Food bars of all types fall into this category, as do granolas, GORP, etc.. They all work well but should be replaced yearly or according to their expiration dates. The Coast Guard uses a food bar with a five year shelf life. Get ones with lots of variety, such as nuts, whole grains, etc… The next best type of food is the mylar type- mylar bags of tuna, MREs (military meals), etc.. They do require utensils, but the tuna (available at the grocer) can be eaten out of the bag, as can the MREs albeit with a bit more difficulty. The excessive amounts of packaging and superfluous items in MREs have always kept me from using them as a serious food source if space and weight are an issue. NOTE: EAT AS HEALTHY AS POSSIBLE. Do not fall for the “take plenty of candy and junk food for comfort and energy” gig. Sugar causes a crash in energy proportional to its high. It also pulls nutirents from bodily stores in order to be digested, and it stresses out the body, and in a stressful situation it is the last thing that should be eaten. Whole grains, nuts, etc. will give gradual, even energy and provide the necessary fats, proteins, and nutrients needed – needs which increase during times of stress.
Bedding. Wool and fleece are the best blanket choices. Wool retains some of its ability to keep you warm even when wet, and is flame retardant, fleece is very lightweight and also retains some of its ability to keep you warm when wet. It is NOT flame retardent. If you are in an area where fire is likely an issue, stick with the wool unless it is just too heavy. Stay away from cotton, as it wicks heat from your body when it is moist. Get orange blankets if you can, this is a lot of surface area for signaling (even if a child is wrapped up in it). I think it is a good idea to include a foil type of emergency blanket. They fold up very small and can act as a ground cloth, it can retain body heat, act as a vapor barrier and wind breaker, etc.. Don't buy all the hype about its amazing heat reflection properties though- it only reflects radiant heat- and it is, after all, just foil. Persons in very cold climates might investigate buying a down sleeping bag for all of your camping and storing it in your 72 hour kit. It packs incredibly small and light and is amazingly warm.
Hygiene. Pack what you need to stay comfortable, but don't overdo it. Toilet paper, cotton bandanna (for face mask, wash cloth, etc), baby wipes, soap, potty bags, and an N95 dust/virus mask are what I think are essentials. Additional important items include a toothbrush/paste, latex gloves, and feminine napkins.
First Aid. DO NOT get a standard first aid kit with 3,200 band aids and a few aspirin. Get a roll of gauze, some gauze pads, medical tape, some painkillers, alcohol wipes, some burn/wound cream, some blood coagulant (such as cayenne, which is also antibiotic), and some petroleum jelly or some other lubricant/moisturizer. You'll have to make your own band aids from gauze and tape, but you'll be able to cover most other minor injuries.
Tools and Rescue Aids. Again, DO NOT over pack. You need light (light sticks or a crank flashlight), warmth (heat packs), and a whistle. Additional ‘might need' items are: Fire starting it, multi-use pliers, small rope, hand crank radio, duct tape, and a folding saw. Generally everything else is superfluous compared with the instances in which it might be needed. If you live in rural Idaho might pack a bit of fishing string and a hook, but don't overdo it.
Now you have to decide what you will pack it all in. A Rubbermaid is easy to access but don't try to carry it anywhere and the lid can come off easily (yes, I have had contents scattered across the highway from the back of a trailer). Dry Bags, duffels, daypacks, etc are all viable options depending on your budget and location. If you want to be able to carry it, and want it to be waterproof (a good idea- keep in mind that most natural disasters in the US are related to water, and wet contents are a surefire way to kill your spirits), a dry bag that can be carried as a back pack is the best option. Many are tough and waterproof, and a few can be carried in several different ways.
The last thing- try it out! Take it camping, use it often. Buy the items as camping gear and store them in your 72 hour kit. You'll get your money out of it and know how it all works when you really need it. Likely, your kit will become that place where you always go when the lights go out, when you can't get that bag of chips open in the car with your teeth, when you are stuck on the side of the road, when you can't find a flashlight anywhere else, and of course when that 3 feet of snow pins you down on a desolate stretch of Iowa highway.