I've never been in a monogamous relationship. But that would seem like a situation where you already have decided on a bound in advance, and so a rule reminding you to set such a bound doesn't move you forward.
Boundaries have a way of getting fuzzy in the moment. Is it cheating if you kiss someone? If you kiss with tongue? If you put hands on breasts? Etc. Slippery slope, and all that.
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!)