Related to your original maxim: If you want to buy something, always spend at least five minutes, by the clock, thinking about whether the you of three hours from now, three days from now, and three years from now will be glad of your purchase.
This is a maxim that I only just now formalized, but it's something that, in principle, I always try to do.
I find it useful to consider alternative uses of the same money.
In a New York shop, I once got pressure-sold something expensive I didn't really want; when I said it cost too much, I was asked what I might be prepared to pay, and we ended up haggling. Since then, I've had a rule:
and I have been very glad of it on many occasions. I can go for a short walk to decide, and if I don't want it, I simply don't return to the shop. This means I'm deciding in calm surroundings, based on what I want rather than on embarrassment.
Are there other maxims I could adopt that would serve me equally well?
(Personal note: I'm in the Bay Area for a week after minicamp, Sunday July 29th to Sunday August 5th. Let's hang out, go to things together, help make my visit cooler! Mail me: paul at ciphergoth.org. Thanks!)