You’ve spent months combing through magazines and visiting dealerships to determine which car you want. But the 30 minutes after you’ve negotiated a fair price is when you really need to be vigilant, says Philip Reed, a coauthor of Strategies for Smart Car Buyers (Edmunds Publications, $15, amazon.com). During that time, a showroom salesman will usually usher you to the office of another salesman, who will press you to purchase important-sounding extras, like fabric protection, rustproofing, paint sealing, additional security devices, and extended warranties. Besides costing you a lot more money than they cost the dealer, these add-ons “tend to be utterly worthless,” says Remar Sutton, author of Don’t Get Taken Every Time: The Ultimate Guide to Buying or Leasing a Car (Penguin, $14, barnesandnoble.com).

There are exceptions: If you’re leasing a car, gap insurance — which covers the gap between the value of your car at the time of an accident and the amount you still owe the bank or leasing company — is a necessity, Reed says, so you’re not liable for that amount. “And if you think you’re going to have your car for longer than its original warranty period, an extended warranty has some value. But just as in the showroom negotiations, there’s a ton of wiggle room when it comes to price.” Weary of wiggling? Sutton suggests you decline all extras and visit your local credit union, where the extended-service programs — and service people — might be more reasonable.