I think a "big mistake" a lot of people make in life is assuming they need to spend a lot of money on certain items like food & clothing. If you work hard at it you really don't need to spend a lot of money on either of those items. Just eating a sandwich brought from home for lunch instead of going out for lunch every day can save you between $30 & $50 a week. Skipping buying a cup or two of coffee each day or a beverage out of a vending machine is another dollar or two a day.
My wife & I spend an average of less than $50 a week on food and we eat very well. We just don't "waste" a lot eating out and make cost saving meals at home. At first it kind of sucks but once you realize all the inexpensive healthy choices you can make when it comes to food you can save a good bit of money each week.
Clothing is the same way - you don't have to do without but a $8 pair of jeans will last just as long as a $50 pair of jeans. Just buy stuff on sale and watch for deals and you can save a boat load of money.
As far as fixed expenses:
Taxes come first because you don't have a choice when it comes to taxes
Then comes Housing, Insurance & Health Care - again very little choice on those
Pretty much everything else you can control and use cost saving measures if you want to give up a little to have some extra cash the end of each week.