4. buy in season vegetables (cheaper)
5. know the regular prizes, so you can detect real discounts from fake ones
6. buy in bulk (cheaper)
7. prepare several days food at once with that bulk and freeze it for later use.
8. skip expensive food (usually meat) on a regular base when on a budget and bij it only as a treat/for weekends

When you buy in season, there is usually a lower prize as it's in abundance as it's grown in the fields instead of greenhouses. I've seen discounts that were the regular prize but the 'original' prize has been increases to make it seem a discount. When you buy in bulk (say in a 2 person household for 4-8 servings) and prepare in bulk you can freeze in 2 serving potions. (saves preparation time and it saves on groceries runs ;) ) Defrost food in the cooler, which takes about a day to defrost and save a tad on energy for the cooler, Also you save on the preparing side as preparing a 4 serving meal doesn't requires a lot less energy then 2x a 2 serving meal. (including the re-heating of the 2nd meal) Save leftovers. Even half portions can be combined with others to make a fuul (and maybe even interesting) meal.

Also, when you eat meat less often you save a tad (when you eat meat, when you don't the biggest saving is already achieved).