One thing to add:
Consider the possibility that the true embarrassment in the client dinner situation was not the card being declined, but the way you handled it.
My standard response when a card gets declined is "Oh, these banks are all so twitchy these days about 'unusual' spending. They must have locked my account over something I bought earlier today. Here, use this card instead." 99% of the time, that's exactly what happened, but even if it's not it saves face for everyone involved.
The waiter or clerk doesn't have to feel like the bearer of shameful news, anyone else in the vicinity doesn't have to feel tense about witnessing the incident, and you have nothing further to explain.
It's only embarrassing if you let it embarrass you.
To put it another away, emotions are not caused by the things that happen to you, but by your own thought processes about what happens.
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