1. Will this make my life better?
Many times we buy things because they are interesting or cool. We need to begin to look at our belongings as tools that need to fill a function. Ask yourself if buying this item will ultimately save time, energy, and/or money. Will having this item eliminate an important need that you've had? If not, then you bay want to think about not making that purchase.
2. Do I already have one or something similar?
Ask yourself if you already own one of these items, or something that can take care of the same job. If you do, then you are doubling your clutter for no reason. Make sure that the need for the item is pressing. It may be possible for you to borrow/rent the item to provide more control of the clutter.
3. Am I ready to get rid of an old item that this replaces?
If you can't part with an older version of something, then you shouldn't buy the newer one yet. Be willing to get rid of the old in order to enjoy the new.
4. Where will this belong in my house?
Do you have space for this item in your home? Where is the specific location that this will be stored? If you don't have a place for it, it's not a good idea to buy it. It's an invitation to clutter up your home. If you don't have a place for it, and still really need or want it, make space before you buy it. Get rid of other items, reorganize, or whatever it takes to create a home for your new item.
5. Perishables: How long will this item be good?
We don't think of perishables as a big threat to our clutter because they'll eventually be thrown out. In order to save space and money, figure out exactly how long it will last. Are you able to finish it before it will go bad? Sometimes a deal on perishables makes you want to stock up. Make sure you are only stocking up on what you'll be able to finish and use.
6. Have I researched price and quality enough?
Hasty decisions are sometimes the worst. Make sure you've researched prices, features, quality and more to make the best informed decision about where to spend your money. If you're not sure, wait. Read reviews. Talk to friends. List pros and cons. Take a little time to look around at your options and make an informed decision.