Too often people forget that it is more than just survival during an emergency - you have to be able to live through it and go on living after it is over.
That is the type of thinking I am always telling people that they need.
Back in 1999, I was one of the local "computer experts", and I constantly had people come up and ask me if I thought they needed a "Y2K Kit". And many were rather surprised when I told them "Yes, you definately need one". Then when asked if I thought we would loose electricity and all the other doomsday predictions, I said "Of course not, but we live in LA, and you need an earthquake kit, which is the same thing." Needless to say, most just laughed and forgot all about it.
In my life I have been through more disasters then I want to think about. 3 major Earthquakes (1971 San Fernando, 1994 Northridge, 1999 Hector Mine), more Hurricanes then I want to remember (including Katrina and Ivan), evacuations for fires and flooding, and 4 days of 10 degree weather without electricity or water.
And in each one of those since I became an adult I relied upon my disaster kit. In 1994 after the shaking stopped, my late fiancee and I were loading up the truck to leave (we lived 3 miles from the epicenter) when the guy that lived next to us asked if we had any water. He had 3 young children (2 in diapers), and no water to make formula with. So I gave him 6 gallons, which was enough to last him the first few days.
If somebody is really serious, such a kit is not hard to make. I worked at a Surplus Store in 1994, and advised a lot of people who decided to prepare them "after the fact". But these are my main pieces of advice:
Keep your main kit in a backpack. This way in case you have to leave on foot, you can bring the essential things with you.
Keep it in your car. I had people who looked dumbfounded when I suggested this, even when I told them that if something happened and they were at work they would still have it with them. Some insisted that their house was a better place, and that walking to their car if it was needed would be to inconvient.
I also encourage people to have flashlights, a radio, and 2 to 3 sets of batteries for each (and a solar battery charger). Forget the hand-crank radios, I have never found one yet that was worth a damn (same with the shake flashlights).
Before I got my RV, I kept all my supplies in 3 seperate kits. One was always in my vehicle, and had enough to get me through at least 3 days. Then there were 2 progressively larger ones in the house, that in total would keep me for a total of almost a month, including a tent and large tarp (in case I had to evacuate such as my house was destroyed), a bucket with a toilet seat and lots of bags to line it with, a folding military shovel, crowbar, gloves, and other things I might use to help extracate others. The larger items were kept shoved into 2 military duffelbags.
I also encourage enough canteens or collapsable water containers so that everybody has at least 1 gallon of water. These take up little room when stored, and will be impossible to find when you really need one.
The thing people need to remember that survival comes in 3 stages. First, is immediately surviving the disaster.
Second, it is getting to someplace safe.
Third, getting by until you can start to put your life back together.
To many people seem to think they can rely on the Government to get them through, bad move. Local governments are often incompetant when it comes to a disaster (look at New Orleans), and it takes on average 2-4 days for the Federal Government to respond. So for that initial time, you are going to be largely on your own.
And do not rely on forraging. First off, it is very hard to "live off the land" unless you are a real expert. Remember, hunter-gatherers usually spent 60-80% of their waking hours gathering food, and they were real experts. Plus, you are not going to be the only one with that idea. And what is to say the disaster is going to happen at a good time for such activities, or that plants and animals survived it as well?
But if you are going to forrage, the best thing you can do is make "eternal soup". Get the largest pot you can (or a small metal garbage can if there are enough in your group), keep it barely at the boiling point, and add in anything of nutrition as needed. Squirrels, domestic animals, roots, veggies, anything you can. Because it is constantly at boiling temperature, it should stay safe to eat. And even things generally considered inedible (rodents, insects, tree bark, pine needles) will add at least some neutrient value, and stretch food supplies longer among more people.