For us, we always break our storage down into 5 categories: Food items; Health items; Hygiene Items; Light/Heat; and Useful items. In an impractical world, each bucket would contain every single item you could ever imagine you would need but realistically that is not possible.
We use 5 gallon round buckets or 3 1/2 gallon square buckets. The 5 gallon buckets you can purchase at Wal-mart or a farm & ranch store. The 3 1/2 gallon square buckets we get for free from bakeries. Most grocery stores have bakeries inside and these bakeries purchase their icings for cakes in 3 1/2 gallon square buckets. Once the icing is used and the buckets are empty, they generally throw them away. Just ask them for them and 90% of the time, they are willing to let you have them.
Here are the items we placed in our newest bucket (3 1/2 gallon size):
For food stuff for this particular bucket, we have 4 slim jims, a large can of beef stew, 6 servings of chili, 1 can vienna bites, a small can of tomato juice, a V-8 smoothie, 1 can of tuna, 4 pkg. of Ramen noodles, 1 pkg. of sunflower seeds, 2 pkgs. of long grain & wild rice with seasonings, 2 small pkgs. of turkey nuggets (dried meat).
Other food suggestions you can put in a bucket could be MRE's, water purification tablets, peanut butter and cheese crackers or beef jerky.
Light and heat items for this bucket are a flashlight, matches, lighter, cotton round soaked in wax and paper strips. The wax cotton round is very useful to have. As you burn a candle, just dip a cotton round (things women use to remove make-up from their face) into the wet wax and let it harden. Once harden, tear the round in half to expose the fibers and when needed, light the fiber. These will burn anywhere from 5-15 minutes which allow you time to really get a fire going good. The paper strips just allow you something else to light in case you can't find dry items, etc.
Other things to put in a bucket in this area could be hand warmers, gloves, stocking caps, & a glow stick.
The useful items we have in this bucket are a small pocket knife, 5 pieces of paper and a pen, 5 zip ties, 2 kitchen sized trash bags, 2 plastic forks, plastic spoon, a pair of 36" shoe laces, 4 sugar packs, 1 sweetener packet, a tea bag, a small jar of instant coffee, 1 package of snus (could be traded), 1 roll of duct tape, vegetable seeds (carrots, pumpkin, peas, bell pepper, tomatoes, radish, cucumbers, lettuce and a 14 variety salad blend), 2 baggies and a baggie full of toys (14 rubber bouncy balls, 3 hot wheel type cars, 3 spinning tops, and 5 small dinosaur toys).
Other useful items you can put in one include paracord, electrical tape, multi-function tool, a deck of cards, can opener, fishing line and hooks, tarps, superglue, sewing kit or a million other things.
I know that this bucket does not have EVERYTHING but it is just not possible. What items I think I may need that is not in this bucket will be in the next one.
And as you can see from the photo below, all items listed fit perfectly & tightly into a 3 1/2 gallon bucket. Now it is ready to be sealed and hidden.