OK - what I mean is, know what you are going to cook for, say, the next seven days. Write it down. I use a table in Word, some use a wipe-able board, some use printable templates, some use a notebook. They all work. Don't just plan dinners, plan the other meals too plus snacks, if you are into snacking. If that's too restrictive for you, plan seven breakfasts, lunches and dinners, then you can choose what you want each day from those lists, knowing you have all the necessary ingredients.
Plan your meals, go through what you need and make your shopping list from that.
And then stick to it!
These are what works for me. If you're serious about reducing what you spend of food and want to do it properly, find what works for you and use it!