Here's something that has helped me out immensely. Let's say that there's a new video game or something that you want, it's relatively expensive at about $50. When you see this game or "want" item ask yourself, "do I reeeaaallly want this now?". If you even have the slightest doubt, wait 2 weeks and ask yourself how bad you want this new thing. I do this to myself all the time, sometimes it gets to the point where I forget about what it was I wanted to begin with.
A second part to add to this is to take that money and put it into a piggy bank while you are deciding during that two weeks. And if this works for you then you will eventually have a bunch of extra money.