We rotate between the big supermarkets and only buy packets/tins/household goods when they're on offer. Most of the big brands are on offer on a kind of rotating carousel between the big supermarkets, so as long as you're not desperate to buy having run out of something, you can get things a lot cheaper just by shopping around and waiting for the offers to come around again. I.e. washing powder, canned drinks, crisps, tinned food, etc.
We also look at the yellow stickers areas and buy/freeze things like meat, cheese, ready meals etc to defrost at a later date.
If you set up online shopping accounts with the big names, you can browse their websites to see what they've got on offer to prepare yourself with a shopping list before you go in store.
It also goes without saying to get and use store loyalty cards, such as Tesco Clubcard, Nectar, Morrisons, Asda rewards etc to get the deals which bring prices down to what they should be in the first place, i.e. without the rampant over-charging!
The big supermarkets and big brands are rip off merchants with stupidly high over-charging for things that aren't on offer that particular week - they're not "normal" prices, they're "mug-pricing", i.e. over-charging in the hope of getting lots of "mugs" who don't care about what they pay or are disorganised and get caught out having to pay more for things they need at that moment.
Play them at their own game!