The following is a guest post from Claire at ChoysterCash.com.

I am a huge fan of tracking your spending, as I believe that the key to financial freedom lies in the record-keeping. Indeed, it wasn’t until I started writing down every single expense I made on a daily basis that I really began to make headway in terms of true financial freedom.

Over the years, though, I have realized that there is one item on your expense list that has the most power to change your spending habits for the better.

The One Expense You Shouldn’t Have Made

I never went out of my way to write down the one thing I shouldn’t have bought in any given week. Indeed, it wasn’t my intention to force-feed buyer’s remorse into my conscience. In the beginning, when I transferred my daily expenses from the small notebook I carry everywhere (I call this my “money book”) to my excel sheet on a monthly basis, I simply took note of things. Oh – that’s interesting. I completely forgot about that $29 belt I picked up at JCrew last week. Huh.

Over time, this exercise in viewing my expenses as they went from hard-bound book to excel chart changed, and I began to evaluate if each purchase I made in a month was indeed something I wanted and was happy to have bought. Did I really enjoy that $8 I spent at Starbucks, or did I just shell out the cash that because I hadn’t brought anything better to eat?

This evaluation soon deepened to the point where I could look at my expenses and immediately see what should have been cut out, or what could have been saved if I had simply planned ahead better. If I had just remembered to pack a lunch that day, I wouldn’t have needed to spend $12 at lunch on Thursday.

I am now convinced that the most important expense you make every week is the one you shouldn’t have made. It is this expense that teaches you more about spending than any other. So what are the two biggest changes you can see in your spending habits as a result of such a practice?

You Learn to Plan Better to Save more

Spending – to eat out, to go to Starbucks, to buy a magazine – can be fun and well worth it . . . if you are enjoying it. But the kind of spending I hate is when I simply have to spend because I didn’t plan. By evaluating which one expense you shouldn’t have made each week, you get better at planning so that you avoid this expense.

Now, I highlight the expense I shouldn’t have made each week. It can be a small one (was that stupid $4 magazine you bought at the airport really needed?) or a big one (the $700 you paid for new seat cushions in the car when they weren’t really “necessary”). Either way (and depending on your financial situation) it helps you see what expenses you need to think more about in the future.

You Learn to Second-Guess Purchases

By thinking more about what you might buy, you end up spending less: Is this going to end up being the one item I most regretted buying all week? If so, maybe I should at least think about it for half an hour. This kind of second-guessing helped me immensely in areas of impulse shopping – clothes, electronics, books, etc. By giving myself one more reason to stop and think, I avoided buying things I might regret later.

If there is one thing you can do this week to change the way you spend money, it’s write down the one thing you shouldn’t have spent money on. Try it – you’re bound to learn something!

Claire writes at ChoysterCash.com. See her site to sign up for a free couponing class or to take part in a $100 giveaway.