You are in the store. You have no idea what you intend to buy. I should back that up a little. You wrote a list. You forgot the list. I do it all the time. I figured out a way to stop leaving the list at home.
The first thing you can do is make a list on your phone. You can install google drive and then open a document and write in there. Then as you get the items on your list you just delete your document.
The second way you can keep your list with you is to write it in your car. This is my grocery list printable. I put it in a sheet protector then I use a vis a vis pen,then I just mark off what I need and cross them out when I buy them.
I also suggest before you even step foot out of your house you look to see what you have on hand. Do not buy what you already have at home.
You know I suggested meal planning as one of the ultimate ways to save. You can read how I do it in this post The Meal Plans You Need. You can see a couple of wonderful meal plans for slow cookers, pressure cookers, and we even have a healthy meal option.