Some people spend their life on a spending fast, but this is for those who have the income and can't hold onto it.
In a spending fast you eat at home, no going out to eat at all; you try to eat out of your cupboards, your freezer and your refrigerator as much as possible, and only buying the absolute minimum foods and supplies needed to get by.
You also don't go shopping; you don't buy any clothes or shoes or jewelry or dvd's, rent movies, or anything that isn't an absolute necessity. There are few of those necessities: shelter, clothing, and food, that's about it.
If you have shampoo in your bathroom for example, you use it up, you don't buy new bottles of shampoo just because you can.
Most people who go on a spending fast pick a month as a reasonable amount of time for it, although even a week or two weeks can make a difference in your spending habits.
The money you save during your spending fast is used to pay down debt or put into savings. It is not just wasted and spent as soon as you go off the spending fast.