Now, some of you are out far enough away from the city that this doesn't apply to you. So take it or leave it, I'll try to be as non-pedantic as possible.
Until the collapse we are probably going to have to deal with the corporate store that brings in the "just in time" vittles. That being said, now is the time to start minimizing the amount you use these facilities and choosing the one that best has the chance of getting you what you need while we are waiting and hoping that the worst doesn't happen.
First, when you go to the store, take a look at the parking lot. If there are a bunch of expensive cars and well dressed people, that is probably not the store for you. If you are still eating high off the hog, you probably ought to reconsider your lifestyle. High off the hog is what is coming to an end. We could live that way as long as we didn't save anything, borrow more than we can afford, and try to keep up with the Joneses. If you are prepping and still spending huge on your day to day supplies, I don't think you will be able to react when the SHTF.
Now, back to the store, who is in it? Well-dressed white folks are a bad sign. The greater likelihood is that they are still "livin' the dream", and as such, they are easy marks for high prices and a lot of useless shit to impress the neighbors. These stores prey on the gullible and foolish, it isn't for you.
Around here, if you get through the parking lot OK, and there is a minority of dumbass whiteboys in the front of the store, start looking for the Russians and the Mexicans. These are harbingers of good things. They keep to the basics, cook at home for families, and the women can usually squeeze a nickel 'til it shits a dime.
So, you have found your store and it has none of the negative indicators (Expensive cars and well-dressed whiteys) and some of the positive indicators (Russians or Mexicans) you have your store.
To fully exploit this store, you will need to do a little discrete stalking. Pick out a women (again, Russian or Mexican) with three or four well-behaved kids. Make a point of the well-behaved data point, if they have kids running around like maniacs, they are not going to teach you anything. Anyway, follow this woman around, note what she buys and try to figure out how here family eats. The ability to isolate the cheapest high-quality carbohydrates and proteins is an acquired skill. This will allow you think about how to go about feeding families as low as possible. Granted, you will be looking at some funny food at first, but you can spice it up your own way and make it into something positive.