I have a few simple things that I do, firstly whenever I decide I want to buy something I always wait a period of time after that initial decision, let's say 2 weeks, if after 2 weeks I still want it then I ask myself why I really need it and if it will improve my life, if the answer is yes then I'll buy it.
That`s a good idea trying to figure out if it is really needed or we can say that it can improve our lives through it. But for me when i`m trying to buy something well i think of it first the quality. Second, if it is really needed or it helps in my work.