#1 - Shopping at a regular grocery store.
Bypass the regular grocery store UNLESS they have what you want on sale as a 'loss leader'. Sometimes these items have purchasing limits, though, so you have to be creative in getting the quantity that you want. Instead, look for what you need at warehouse stores. Don't just go to Costco and Sams Club. Look at places like Grocery Outlet or Cash-n-Carry or Smart-n-Final. Also, buy direct from suppliers when you can. That means contacting wholesalers (look in the phone book) to see if they will sell you case lots. Sometimes you have to organize a buying co-op and split an order among the group to get the best deal. If you're buying by the case, NEGOTIATE the price. You should get a discount for a large order. Lastly, contact your local LDS community to see if you can join in on a canning day at a local food processor. For working in the factory, you often get a greatly reduced price on the item they are making that day. Sometimes your local University Extension Office will also have information about these events.
#2 - Buying unusual foods like MRE's or pre-packaged emergency buckets.
Buy ONLY what you're going to eat or use. Buy basics and only what is a necessity. Don't stockpile optional items. Buy items that can be stored for a long time or only buy what you can use before it expires. (And yes, toothpaste expires.) Buy items in container sizes that you can use. It is no use opening up a gallon of olives unless your family will eat a gallon of olives in, say, a week.
#3 - Not storing food appropriately.
Don't expose canned foods to temperature extremes or high humidity. The garage is not usually a good place for food storage. The same usually goes for the laundry room. Be creative but keep an inventory! Store canned goods under beds or stack cases and cover them with a tablecloth to make end tables. Store lighter weight items in the closet space above the top shelf. Use any dead space you can find.
#4 - Relying on the freezer.
Try to avoid stockpiling food that needs to be refrigerated or frozen. Freezing food is an incredibly expensive way to store food. Save the freezer for really expensive food items like meat that you can use soon. Can the harvest! Even meat can be canned at home with a pressure canner. Dehydrate foods, too. That is an incredibly compact way to store food. Vacuum pack shelf stable foods.
#5 - Not allowing for shrinkage.
We did a great job of shelving our stored foods. Water was on the floor, fragile jarred items were on the shelf above. If they fell off the shelf, most of the jars would survive unbroken. The plan worked, too, until the '89 Loma Prieta earthquake when the food in glass jars fell off the shelves and the shelves of heavy canned foods fell on top of them. Oops! In many emergencies, there is a chance you'll lose part of your long term storage. Count on 10 to 20%. Also, watch for rodent and bug infestations. Prepare ahead of time to discourage them. We lay down sticky trays so we get an indication of what is crawling around when we're not looking. If a critter gets in, deal with it immediately.
#6 - Not rotating your inventory.
DATE your canned goods so you can rotate the stock. If you are going to donate the item, you can remove permanent marker with rubbing alcohol. USE what you store in everyday eating. Whatever forces us to eat exclusively from the stockpile is usually a stressful situation. Don't make it worse by completely changing the family's diet. Know how to use the basics. For instance, I must have over a dozen recipes for making foods out of powdered milk. I can even make cheese with it.
#7 - Not including a broad range of vitamin, mineral and fiber rich foods, especially vitamin C.
Humans need vitamin C everyday. Most long term food store is woefully amiss in vitamin C. We usually get enough protein or could live on less, but a diet low in fiber will be painful. And a diet that is suddenly high in fiber can be worse. Some people store multi-vitamin or mineral pills as a little extra insurance.
#8 - Not storing comfort foods.
For some people this would be chocolate. For others it would be coffee or alcohol. If you smoke, quit. If you can't quit store some tobacco. Keep in mind that some of these comfort foods can be used for barter.
#9 - Not storing emergency non-food items.
Even if you don't regularly eat off paper plates or use a paper towel, if there is no running water, the disposables will become a necessity. The same goes for needing things like wet wipes and large plastic garbage bags. Make sure you have supplies for sanitation.
#10 - Not having the proper tools.
If all you have is an electric can opener, your canned goods are useless if the electricity goes out. Can you cook your food if you don't have power to the stove or microwave?
#11 - Under-estimating calorie needs during an emergency.
Usually an emergency requires us to increase our physical activity significantly. Just a little thing like having to walk or bike everywhere instead of use the car will change our calorie requirements dramatically.
#12 - Under-estimate the need for water.
Water isn't just for drinking! You'll need to wash with it - both personally and objects. You'll need water for cooking, too. While I tend to use rice a lot, I can only really use it when water is freely available.
#13 - Not preparing for medical problems.
If you have chronic medical problems, be sure to stockpile your regular medications, too. Work with your doctor on this. Sometimes they'll give you samples to keep at home or write your prescription for double what you actually need. Mail order pharmacies are cheaper and will usually send you 3 months of medications at a time. Basic anti-biotics can be bought through veterinary supply houses - and yes, it is the same stuff that humans take. Just make sure you're informed on dosing requirements.
#14 - Not preparing on pet needs.
If your pets are members of the family - and some will provide extra security, too - you have to feed them. In the same way you don't want to be suddenly changing your diet, you don't want to suddenly change your pet's diet either.