I start with do I really need this? Often times my wants are disguised as needs. As such, I try not to make purchases on a whim. I'll usually carefully consider the purchase for a week or more before pulling the trigger. If I determine that I need something, I weigh up how good the quality needs to be. If it's something that I will use regularly, I buy the best quality item that I can afford. I also try to buy used as much as possible.