Needs, wants, and not even wants
Hang around with frugality and simplicity types for any length of time and you'll hear a lot about distinguishing between needs and wants. It's come to me, though, that this issue is less interesting than the much more important issue of distinguishing between your wants and those brief, transitory fancies that don't even rise to the level of being true wants.
It's easy to see the "not even" wants after the fact: They're the things that you buy, play with for a day or a week, and then set aside to add to your clutter. For some people they're electronic gadgets. For other people they're toys or shoes or clothes or sports equipment.
Personally, I find it easy to do this with tools of a creative nature--painting and drawing supplies, musical instruments, and so on. It usually happens like this:
I see a work of art that speaks to me; one where I'm impressed not by the virtuosity of the artist's skill but by the way the image captures something in my own experience. When that happens, it occurs to me that I could do my own drawing or painting of a similar subject or on a similar theme. If I followed up by doing some drawing or painting, all would be well. But too often I follow up by buying some paint or ink or paper: stuff I not only didn't need, but that I really didn't even want.
For other people it happens different ways. They buy something because a coworker got one, or because friend praised the thing, or because a mentor told them they needed one, or because a new boyfriend was shocked to hear that they didn't already have one, or because an ex-girlfriend mentioned how cool the thing was, or because a child threw a tantrum, or because a spouse gazed wistfully at one.
The whole purpose of advertising is to produce this reaction: To turn something you don't need (or even really want) into something that have to have--for long enough to get you to make the buy.
I don't have much new to say about resisting your "not even" wants. You've heard a hundred times about waiting a few days and seeing if you still want it.
The classic simplicity book Your Money or Your Life suggests translating all prices into "life energy"--the number of hours you have to work to earn the money to buy the thing. (Be sure to include the extra hours you work to pay the taxes and a share of the hours you spend commuting and the hours you work to buy the work clothes and the hours you spend shopping for work clothes...) If it's worth that, then by all means buy it.
What's worked best for me is to spend some time looking at the things I already own and thinking about why this new thing might be more like the well-worn precious things I use all the time and less like the barely used, never used, no-longer used things that clutter up my apartment.
In fact, it's worth doing that now and then even when you're not thinking about buying something new. After all, setting aside gifts, everything you own is something that you managed to convince yourself, at least for a moment, was something that you wanted. Go through that stuff. Even if all you can think is "What was I thinking?" there's a small education in that. Sometimes, though, you can think, "Wow! That's so cool!" and get it out and use it.
I think I'll go through my art supplies tomorrow, and maybe make some art.