I once read a really interesting book about the techniques used by grocery stores to encourage us to spend more money. One of the most surprising things I learned is that stores love shoppers who work with a list. According to this source, if you come in with a list, you won't be satisfied until you've purchased everything on it. In other words, even though you're feeling so organized, what really happens (to all but those with steely self-discipline) is that you're on a mission; you'll go all over the store if necessary until you've checked off every item. Along the way, you're sure to pick up sale items, that deli salad you can serve with dinner, the great smelling fresh bread, those new products on the end caps, and the tasty sample item in aisle 9. The longer you stay in the store, the more you're going to spend.
One thing I'd suggest is this: next time you're at your primary grocery store, stop at the customer service desk and ask for a store map. From there, consider the things you typically buy and plot out their location. Make a personalized checklist based on this information and keep a copy handy as you do your meal planning and run out of staples.
I've done this for several years (with varying periods of commitment) and am certain that when I plan meals and use this list to shop, not only will I have the items I need on hand for every meal, but we eat more balanced meals and I spend far less than I do on the less organized trips.
My list is just a four-columned Word document, easily produced and edited. Mine can help get you started, but be kind - I'm hoping you won't judge me for having Velveeta and tater tots on it. ;-)