In the same vein, never be the sole arbiter of expensive purchasing decisions; pick another person to discuss the decision with. If you're married or dating, you have a good choice there.
This has several advantages; one which may come in particular handy for you in high-pressure situations, is that you can honestly say that you have to discuss the purchase with somebody else. Another is that discussing the issue forces you to articulate your reasons, which will give you a clearer idea of whether or not you really want it.
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!)