Keep a list of what you have bought, where you have put it and if possible the use by or BB dates.
Write dates on top/side of tins jars etc with a Sharpie.
Utilise different storage spaces: people have suggested: under bed, under/ behind sofa, on top of cupboards ( chocolate melts up there though in hot weather)under kitchen units behind kickboards in carrier bags. ( Use the loft hatch hook to hook out again) under the dining room table, down the side of furniture, behind books on the book shelf or bath panel if you can remove it easily. Bottom of wardrobe. Understairs, in loft.
Some things can be stored in the garage. Eg laundry detergent, washing up liquid, cleaning things.
Powdered milk for coffee tastes just as good as fresh milk to me.
If you get chance, work out what you eat in a week and use that to get proportions of the stuff you need.
Think about Christmas presents and food. Cranberry sauce and stuffing have long dates. Parsnips,carrots and Brussels can all be frozen. I plan to buy and freeze a chicken nearer the time.
Don't forget cellotape and wrapping paper.
Have a spare tin opener or two.
Don't forget batteries for smoke alarms and other useful things.
Wind up torches, lanterns radios and chargers may be useful in case of power cuts in bad weather.
keep the petrol tank full, not below half according to Bellini. just done mine
Have things to help keep warm in winter. There are useful threads on Mumsnet every winter. Useful money saving ideas and ways to keep warm.
See medicine first aid kit thread on preppers board, also the non food preps thread. must get cannestan
Work out what nutrients you need and where you can get them from. Eg nuts, seeds, or dried beans and pulses etc. Iodine is tricky if no milk or seafood. You may need different things for different family members according to taste. Dried fruit only needs 30g per portion. Maybe think about meds for constipation?
If you have kids who will only eat one brand of something, get plenty of that in.
Think of school supplies, here we use fifty billion set squares a week. If kids are going to secondary, get spares. Lots of spares. They lose lots.
Buy things that will make storecupboard meals taste more interesting, Eg sauces, spices, mustards, herbs, anchovies, capers, salt, pepper balsamic vinegar, garlic paste, chilli paste etc, etc etc. You can get frozen garlic and ginger or jars or tubes.
Carbs: think about bread flour, part baked bread, wraps, tortillas, naan breads, crackers of various sorts, rice,pasta etc. Pasta lasts well past its BB dates. Spaghetti packs are flat and more dense than other pastas and can fit in smaller spaces.
Look out for offers in the supermarkets and buy in bulk while cheap.
Buy seeds and compost to grow your own. And pots for the windowsill if necessary. Fresh herbs, salad leaves, spinach have been recommended. Think about plant food and other garden supplies, Eg canes?
Have alternative means of cooking. See the annual news story of someone cooking their Christmas turkey on the bsrbeque.