By Becky Quick, contributor

FORTUNE — When I waited tables as a teenager, I learned one lesson very quickly: Treat every person in a dinner party equally, because you never know who’s picking up the bill — and therefore determining your tip. It’s the most basic of business survival tips, but one that few car salesmen in this country seem to have learned. And if you doubt me on this point, try walking into a car dealership seven months pregnant with your husband and two kids, and see who the salesman approaches first.

As the birth of our third child approached, my husband and I decided we needed a bigger car. It was to be my primary car, and after doing my research, all I wanted to do before plunking down my money was test-drive the Town & Country, the Odyssey, and the Sienna minivans. But getting someone to take $40,000 from you can be tougher than you might think, as I learned at one Chrysler, one Honda, and three Toyota (TM) dealerships.

The scenarios all went something like what happened at a nearby Toyota dealership, where I walked to the front desk and asked to have someone show me the Sienna. A salesman came right out and introduced himself to the man who happened to be standing behind me. After the bystander made clear that he wasn’t my husband, the salesman asked me where my husband was — still without introducing himself or asking my name. The sales rep then went on to respond to questions I asked about the Sienna by looking at my husband and talking to him, until my husband told him to talk to me. When the guy took me to his desk to take down my information, he asked me for my home phone number and followed up with: “Obviously you don’t have a work phone.”

Lest you think that I am just particularly unlucky, let me share the story of another woman, Anne Mulcahy, the former chairman and CEO of Xerox (XRX). Three years ago Mulcahy decided it was time to treat herself and went shopping for a Porsche. After test-driving one beauty — a 911 Cabriolet — she announced to the salesman that she’d take the car. After a pregnant pause, he responded, “Don’t you have to talk to someone about that first?” Her reply: “If you don’t start working on the paperwork in the next 10 seconds, I’ll drive 30 minutes to the next Porsche dealer and buy the car there.”

The Porsche salesman got right on the paperwork, but the insults didn’t end there. The finance officer followed up by asking Mulcahy if she needed someone to co-sign on her lease. For the head of a Fortune 500 company, the experience came as a jolt. “It threw me back to my twenties when I’d go for a loan. I was working and was totally financially secure, but I’d have to get co-signatures on my loans just because it was the 1970s,” says Mulcahy. “Then it was discrimination. Now it’s just stupidity.”

It doesn’t take an MBA to recognize the bad business practices on display. Women were the primary buyers of more than 44% of all vehicles last year, and they influenced almost 80% of all auto sales, according to CNW Research. That’s no secret to the auto companies, and any executive would blanch to hear stories like these. But somehow the lesson hasn’t trickled down to the sales force.

The issue may be that the problem is so pervasive in the auto industry. Every salesman I dealt with on my minivan adventure automatically deferred to my husband. And in the end, even a stupid salesman can make a sale when he’s selling an essential good. I ended up buying the Sienna from the dealership I described above, mostly because the baby was coming soon and I was tired of shopping around.

That’s not to say there isn’t an opportunity cost. After an experience like this, I am in no rush to step back into a showroom anytime soon. Which brings me to my memo to any auto company exec willing to listen: If you want a leg up on your competition, teach your sales force the lesson of waiting tables. Treat every customer with the same respect, because you never know who’s going to pay the bill. And that’s my tip.

This article is from the February 27, 2012 issue of Fortune.