"How much life insurance do you have?"
Zig Ziglar liked to say that with that one question, you could tell if someone was a successful life insurance agent. If they're not willing to buy it with their own money, how can they honestly persuade someone else to do so?
If you're in the music business but you never buy tickets or downloads, can you really empathize with the people you're selling to?
My favorite: if you work for a non-profit and you don't give money to charity, what exactly are you doing in this job? I've met some incredibly generous people in the charitable world, but I can also report that a huge number of people—even on the fundraising side—would happily cross the street and risk a beating in order to avoid giving $100 to a cause that's not their own. And the shame of it is that this inaction on their part keeps them from experiencing the very emotion that they try so hard to sell.
Money is more than a transfer of value. It's a statement of belief. An ad agency that won't buy ads, a consultant who won't buy consulting, and a waiter who doesn't tip big—it's a sign, and not a good one.