Buy what you would buy if you were the only person on Earth.
Thought
We buy things for all kinds of reasons. I think buying everything (or even doing everything) due to extrinsic motivation is not an optimal behavior in the long term.
We are Status-Seeking Monkeys. We might buy things to increase our social capital, to impress others, or to seek status. These are all not intrinsic motivations for purchasing something. If we mainly rely on extrinsic motivation, we are handing over our control of evaluating the value of things to other people. In other words, we give up full control of our own feelings.
A heuristic for this is that whenever we want to buy something, we first ask ourselves: would I buy this if I were the only person on Earth. Apparently, there are some exceptions (I won't buy a ping pong paddle if no one can play with me). However, this heuristic serves as a great filter for me in terms of buying stuff.