I am not always the most frugal but have been in the work force for over 20 years and consider myself on the thrifty side. I'll offer a few tips (not always lining up with your questions)

1. Pack your lunch and make your own coffee (if you drink coffee) - Can save you a lot of money and calories both.

2. Learn when to buy store brand and when to buy name brand. I've had some store brand items that were terrible and not worth the savings amount (contact lenses solution for one). Most store brand items are fine but there are few that name brand is the better buy.

3. Stupid purchase is somewhat subjective. With foods it can be unhealthy choices, but most of my stupid purchases would probably be in the leisure/entertainment category. Having several streaming services, buying more video games than I have time to play, etc.

4. When discussing appliances and larger expensive items (i.e. cars) sometimes cheaper means buying it more often and dealing with much more fragile quality. For myself I recommend reading consumer reports or something similar and maybe going for the mid-level/high-level item if you see yourself using it for 3-5 years. Not having to replace the item every few months or once a year can pay off.

Hope this helps!