Some fantastic advice above, I'd say planning & the freezer are most definitely your friends!
Plan based on what you already have, then each week when you shop, plan first then buy ingredients. This doesn't have to be militant but by having a rough idea what you want to eat you can ensure you don't forget to pick stuff up.
Frozen veg can be a total lifesaver too- cheaper to buy and doesn't go off, win win! Great to make a soup with if you need the freezer space back too.
Fruit is trickier if like me you aren't keen on the frozen berries, but I find bananas and apples have a long enough shelf life, and grapes in the fridge last ages too.
Hope this helps!