Whenever you buy something new, get rid of something you already have. If you KNOW ahead of time that you will be getting rid of something you already have in order to make room for the something new you are looking at, you will really think twice before buying it.
This only works if you follow the rule consistently. You have to KNOW that each purchase will require getting rid of something. Once you train yourself, your house will become less cluttered, and you will save money! Tip: each time you decide NOT to buy, put the $ you would have spent in a piggy!