It's more of a generalized rule but:

Assume that your own time has value.

A lot of "frugal" tips operate off the assumption that you can spend your own time and it doesn't cost anything. But your time is valuable. Time spent trying to save a few bucks should be considered working time; ask yourself how much you would get paid by your job for the same amount of time. Maybe you enjoy doing whatever the thing is, so it can be considered recreation, but if it's some difficult or mind numbing slog, then that doesn't necessarily mean that you actually saved yourself anything, because you weren't getting paid to do work, and you could have been doing something more rewarding instead.

Businesses have caught up and fixed the exploits.

For me, it's dried beans. Beans are an excellent source of protein and fiber, and it doesn't get much cheaper per serving than bulk dried beans.

But rinsing, soaking over night, and then boiling, only to end up with way more beans than we will consume, and canned beans are almost as good and almost as cheap.

Searching for the cheapest gas station. Too much time and gas.

Cloth nappies/diapers. Cleaning them is a black hole for personal time.

Flights with connections. Flying has become so tedious, frustrating, stressful, that saving money by spending yet more hours dealing with it, just isn’t worth it. I’d sooner cancel the trip

This is a little different from the others on this list, but a lot of DIY stuff for parties/weddings. The money you're saving is negated by time lost, not to mention unless you have unlimited time/ no job and are able to thrift everything, the components for DIY aren't that inexpensive. For my sister's wedding, we did everything ourselves. Everything from literally painting the venue, collecting/creating every table scape, my dad built the stage and dance floor, all the way through setting up the hundreds of little desserts on the day of. It was all wonderful and lovely. And took a massive amount of time and labor across several families. When you factor in the value of people's time, it was not less expensive than mine. We rented everything and it was so nice not to stress about dressing every corner of the room or decorating the bar, or making sure we didn't run out of ice on the day of. We still chose super frugal options because the wedding industry is a scam, but we just paid for everything.

Sometimes it's worth it just to pay people to do stuff. Value your time and mental health more than money.

Not sure if this counts, per se, but Solar Panels. Specifically, via a loan.

My electric bill is insane, thanks to the powers of capitalism and monopoly. So I figured installing solar panels would be a good investment. Sure it takes ten years to break even, but I’d rather be paying my way through that than paying my electric utility.

Well, the problem I ran into was that the interest on a loan would effectively negate any headway I was hoping to make per month.

I still plan on doing solar, but not before either interest rates at least quarter themselves or I save up enough to practically pay for it up front.

Growing your own food. The only way to make that shit pay is to groom a cult to do it for you, large-scale.

Cutting sponges in half. It just makes them harder to use, and then already last a long time and cost like $1 each. I'm not going out of my way to save ~$1/month.

Unplugging electronics. I have a kill-a-watt meter and did some math. It took more power for my computer to run the spreadsheet than I'd save by unplugging everything in my house. Electronics have gotten way better at managing phantom power draw.

And I'll second coupons. The only coupons I look at is the monthly Costco mailer, and I only really look at things I'll buy in bulk. I try to buy enough to last until the next sale, which has worked out pretty well so far. But I literally don't look at any other grocery store coupons because I just don't find much value there.

In fact, most of these frugal "tricks" are worthless. Just focus on the high value lifestyle choices (cooking at home instead of prepared meals, learning to DIY common repairs, etc), and ignore most of penny pinching. In other words, don't be penny wise and pound foolish.

That said, here are a couple of things that I do think are worthwhile even if the money savings isn't huge:

• cut my own hair - takes 15-20 min once a month, which is less time than I'd spend getting to and from the barber; it's essentially free ($20-30 for clippers, which I've used for dozens of hair cuts), but $20/month saved isn't why I do it, I just hate going to the barber, it just seems to take so much time
• change my car's oil - same as hair, it takes ~30 min, and most of that time I'm just sitting inside waiting for oil to drain; I don't save much money, but I do feel like I save time vs driving to/from the oil change place, and I use high equality OEM filters

The cheapest food is always a massive rip off. It doesn't matter if you're willing to settle for something that doesn't taste as good. The cheapest food has been stripped down to such nothingness that you need to eat 3x more to stay alive. It doesn't work for the same reason you can't just drink water and feel full.

The cheapest option is always cheapest for a reason. Incrementally so the amount cheaper it is than the average.

I always thought couponing looked obnoxious.