I did this for a week and while I didn't spend less, I was more aware of how much I was spending. Unfortunately (for the purposes of this activity - not in general!) I'm quite a frugal shopper in my day to day life so there wasn't much fat to trim. I was glad for the increased awareness of my spending habits it gave me, however.
I moved to Germany two years ago (before that I lived in the US) and one of the first things I've noticed was that most people here don't use credit cards for everyday shopping and normal payments like at restaurants and bars. People do use debit cards for larger amounts, but for the most part it's usually the case that if you go out with a group of friends everyone will pay cash. It's interesting to see the difference between here and in countries where paying with credit card is more common, like the US and also some other European countries. Now that I've gotten used to the way things are here, I've started to think that it was "weird" that in the US I used to pay for a cup of coffee with credit card. And I find that paying with cash helps make how much you spend feel more real in real-time (no need to wait till the end of the month to see the statement), which probably helps people not spend beyond their means.
I don't always pay with cash, but I do try to when grabbing a coffee from the shop downstairs from my office. The owner knows everyone by name and will even modify the menu for people by request (there's a "Jake's Burrito" and "Rebecca-style smoothie" on the secret menu) so I feel badly making her pay any credit card fees. She's so sweet and keeps prices as low as she can manage.
Cash tips are always preferred when going to restaurants and paying for services. As a bartender, I appreciate it because it's money I don't have to claim. But when I got out, I use cash for me. Sometimes, I like to get crazy and buy drinks. When you go out with cash you're limited to that amount and it could stop you from over spending.