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How to prevent myself from buying things I don't want
There are a lot of good answers above, all of them will probably work for you in some way or another. One point to note (from the procrastination theme) is that you could invest your free money that you have currently in some investment instrument which would require you to do some paperwork etc. to get out, this way the immediate cash flow is decreased and also invested. Now from each montly budget save a small amount for the things that you would like to buy. Give this small savings some months to accumulate so that you can afford only one of the items that you want to buy or target an item that you want to buy. After the money is accumulated, if you still want to buy the item, then you probably should. One point of note is that budgeting is also important on a monthly basis, Pete has provided excellent suggestion in this regard.