(MONEY Magazine) – The last time I test-drove a car, I pulled out of the parking lot, made a right turn onto a four-lane road, stopped at a few lights, took a three-minute spin on the highway (passing no one), tuned the radio to Z100 to mollify my son in the back seat, turned around and headed back. The whole thing took about 15 minutes.

A couple of days later, I bought the car. And though I've been extremely happy with it, I have to credit dumb luck. When it came to that test drive, I did just about everything wrong. That's what I learned when I recently took one of the 24-hour test drives that GM is now offering.

Here's how it works: You make an appointment at a dealership to test-drive a particular car. (Not all dealerships are participating--75% of GM dealers are involved--nor are all cars available. You can't check out Hummers or Cadillacs that are in high demand.) When you get to the dealership, you fill out some paperwork. The dealer runs a credit check to make sure you're a viable buyer and a good risk, and takes a copy of your driver's license. You leave your own car as collateral and head off with a full tank of gas, though you may not be able to take the car more than 150 miles from the dealership. At Arroway Chevrolet & Saab in Katonah, N.Y., where I took out a Chevrolet Equinox, they even wash your old car.

Why would a car company take such a risk? Because incentives have gotten so ridiculous that something has to be done to shift buyers' focus from price back to product. Dealers believe that if they can get you into a car for a full day, you'll want to keep it. "Particularly when the brand suffers from perception lagging reality in terms of vehicle quality, as in the case of GM," says Bob Kurilko, vice president of marketing for Edmunds.com, "the best thing you can do is get the product into the consumer's hands." Arroway's general manager, Lou Roberti, says he converts more than half of 24-hour testers vs. one-third of regular test drivers.

Used properly, a full-day test drive can also be a boon for you, says Kristin Varela, who road-tests cars for her website, MotherProof.com. Here's how to take advantage of your 24 hours.

MIMIC YOUR LIFE

If you have a baby, see how easy it is to get the car seat in and out. If you want a pull-down DVD player to entertain the kids, see if it blocks your vision. If you almost always have kids in the back seat, check that there are hooks elsewhere to hang your dry cleaning. If your spouse and your kids play ice hockey, or you shop for a family of eight, be sure the cargo space will suffice. Finally, park the car in your garage to make certain that it fits.

DRIVE WITH NO DISTRACTIONS

For at least part of the day, turn off the radio, leave the cell phone at home and get a sitter for the kids. Then drive highways and rural roads, straightaways and curves. Pass to get a sense of the car's acceleration. Stop short. It wasn't until I'd been out and about for a few hours that I noticed things I didn't like. I made a U-turn through the parking lot of my local gas station--something I do in a single turn of the wheel almost every day. The new Chevy couldn't make it in one swing.

USE THE NIGHT

Drive around after dark to gauge the visibility of the control panel. "Pay close attention to the cruise-control buttons on the front of the wheel," Varela says. "Ones that don't illuminate are a real pet peeve of mine."

Finally, when your 24 hours are up, return the car and say "Thank you," nothing more. Even if you loved it--especially if you loved it--you should take a breather before negotiating. There's nothing a car dealer likes better than negotiating when that new car smell has gone to your head.

Editor-at-large Jean Chatzky appears regularly on NBC's Today. Contact her by e-mail at moneytalk@moneymail.com.

ADDITIONAL REPORTING BY CAROLYN BIGDA