I think this was my very first frugal practice. I started doing this when I was in graduate school, living on my $300.00 monthly stipend, of which rent took about half. Yes, this was a long time ago, but it was still miserable. My friend in chemistry got more than twice as much. Then, as now, the humanities were not valued very much.
You can do this no matter how poor you are. When you buy something, always buy the next one if it is a good price. In graduate school, I used to buy an extra pound of dried beans, thinking, at least I won't starve to death.
I was reminded of this the other day. I woke up first and went into the kitchen. I was delighted that Mr. FS had set up the coffee for me, putting coffee into our French press and filling the electric kettle. Then I noticed a big crack in the pot.
Never fear: I had a back up French press, picked up at the thrift store for $2.00. Now I will be on the lookout for another one.
Similarly, Hunts Tomato company seems to have overproduced 28 oz cans of whole tomatoes with basil. These are 2/$1.00 at two of my favorite haunts: Big Lots and Dollar Tree. I bought a bunch, and would have gotten more except that I am overstocked on canned tomatoes. Even if you are a starving student, you can buy an extra can or two of tomatoes. Then your next tomato sauce will be cheaper than otherwise and you can either sigh happily or spend the extra $0.50 on a few mushrooms from the bulk section.
If you persevere in this practice, you will have a full pantry, which will save time and money.
Other good candidates: socks for kids and birthday gifts.
What do you stock up on?