Not everyone can live in a van like Ieatfish, but here are some tips and dirty tricks that you can use in any living situation to extend the value of your money:
Be open-minded in the grocery store
Most supermarkets have a shopper's card that you can sign up for to receive special prices. Signing up is a no-brainer, but also you want to shop based on what's marked down. You don't want to go in there thinking "I need Oscar Meyer bacon" or even "I need bacon". You go in there thinking you need best value, and make decisions according to what your options are.
Some items are perpetually marked down, so the "special" price is just the price. But often you'll get legit value based on overstock or wanting to push a product or whatever black magic goes into it. If you're flexible about when you eat what, you end up paying less for the same products. The only items I buy stubbornly are honey nut Cheerios and canned sardines.
Manipulate the self-checkout
When you select a produce or bakery item, look at the prices of other items and plan what you'll ring it in as (they aren't naive enough to list prices on the self-checkout screen, so you have to plan it). You can conceivably ring anything in as anything, I think. The machine checks for weight, but it would be news to me if it could tell a carrot from a donut.
But you want to stick to what you're comfortable with. In the off-chance you get caught, it's easy to say you thought a croissant was a roll, but are you willing to say you thought the mozarella cheese from the fancy olive bar spread was bananas? So it's up to you how far you push it.
Save your McDonald's cup
When you buy a drink from McDonald's (or whatever fast food establishment), keep your cup. You can bring it in with you and use it again. It's revolutionary when you realize you can do this.
I'm not saying you're expressly allowed to do it, just that the people working there are highly unlikely to notice and care, so you can physically do it. It may even be unspoken policy to let you get away with it, because the 8 cents they lose on soda is worth drawing you back in to buy food. They'd prefer you paid them $1.79 for it, but to the extent you aren't that gullible, they'd still rather have you than chase you away to Burger King.
Eventually the ink on the cup starts to fade and from time to time they change the design, so you'll need to stay current. But really you could walk in there and fill up a water balloon and still be a favorite to make it out without any hassle.
A softer version of this trick is to ask for a water cup and fill it with soda. But then you have a dinky little cup, and if anything it's more likely to be called out. I prefer to plan ahead and bring my own my cup.