This is from my favorite finance blog Getrichslowly. I find this financial advice effective and a huge help in making my decisions on new purchases.
The 30-day rule is a simple method to control impulse spending. Here’s how it works:
1.Whenever you feel the urge to splurge — whether it’s for new shoes, a new videogame, or a new car — force yourself to stop. If you’re already holding the item, put it back. Leave the store.
2.When you get home, take a piece of paper and write down the name of the item, the store where you found it, and the price. Also write down the date.
3.Now post this note someplace obvious: a calendar, the fridge, a bulletin board. (I use a text file on my computer.)
4.For the next thirty days, think whether you really want the item, but do not buy it.
5.If, at the end of a month, the urge is still there, then consider purchasing it. (But do not use credit to do so.)
That’s all there is to it. But it’s surprisingly effective. The 30-day rule works especially well because you aren’t actually denying yourself — you’re simply delaying gratification. This rule has another advantage: it gives you a chance to research the item you want to purchase. This can save you from grief.
Source
