The auto association estimates the typical medium-sized sedan costs 56.2 cents a mile to operate, or $8,430 a year assuming 15,000 miles driven annually. The estimate is based on fuel prices of $2.603 a gallon and finance charges of $803 a year, based on a five-year loan at 6%. Your costs may well be higher; for example, bad credit can send your finance costs soaring.

Assuming they can afford a payment simply because a lender approved it. Lending standards are much tighter than they were before the credit crunch, but it's still possible to get in over your head, particularly if you have good credit scores and a steady income.

Acting like a lamb before the slaughter in car dealerships. If ever a transaction were designed to soak the consumer, it's the typical car purchase. Car dealerships are experts at getting people to pay too much for cars or financing, and often both. The savvy consumer does plenty of research, gets detailed car price reports from Consumer Reports, MSN Autos or other sources, knows her credit scores and arranges financing in advance -- in other words, not at the dealership. She also knows the tricks of the car sales trade and how to negotiate a good bargain. As any car salesperson will tell you, this describes a pretty small slice of the folks who actually buy cars. Many people wander onto the lot with little information, fall for the salesperson's assurances that they're about to miss out on a great deal, have no idea what interest rates they should qualify for and accept whatever payment the dealership gives them.

What to do now

If you're struggling to afford your car, you've got a few options:

You may be able to sell your car if you have some equity in it or can come up with cash to pay off the loan -- or if you can persuade the lender to let you pay off the remaining debt over time.

You may be able to refinance if you bought the car new, made payments for a few years and have a lender who is willing to extend your loan term. For used cars, you'll need to have some equity in order to refinance.

You can let the lender repossess the car. This option should be avoided if at all possible because repossession, voluntary or otherwise, trashes your credit and usually leaves you with substantial debt besides. You'd still owe the difference between your loan balance and whatever the car brought at auction, plus substantial repossession and auction fees.

You can drive out of the loan. This is usually the best solution for a too-expensive, upside-down car. Trim other areas of your budget so you can make the payments and keep the car until it's paid off . . . then keep driving it until you've saved up enough to buy another car or at least make a substantial down payment.

You also can try to reduce your transportation costs by driving less, raising your insurance deductibles and maintaining your car properly to avoid big repair bills.

Next time, be smart

Use a bad car ownership experience as motivation for change. The next time you're in the market for a car:

Rethink the whole thing. Isn't there something else on which you'd rather spend nearly $9,000 a year? With that as your motivator, you may be able to find a way to live without a car, or with one less car if yours is a multiple-vehicle family, or to keep the car you have going for a little longer. Maybe not, but it's worth thinking about the options before you commit yourself to another payment.

Figure out what you can actually afford. Thompson, the Jacksonville credit counselor, says total car costs -- including car payments, fuel, insurance and maintenance -- shouldn't top 20% of net (after-tax) income. McGeary's Pennsylvania agency uses a slightly different rule of thumb: Total car costs shouldn't exceed 19% of gross (pretax) income.

These are only guidelines. If you don't have other large expenses, you might get away with higher car costs. If you've got a big mortgage or child care bills to pay, you might need to spend less.

I like the overall budget guidelines in Elizabeth Warren's book "All Your Worth." The Harvard University professor and bankruptcy researcher recommends that all your basic expenses -- shelter, food, insurance, child care, transportation and minimum loan payments -- total no more than 50% of your after-tax income. That ensures you'll have enough left over for variable expenses such as clothing and vacations (30% of after-tax pay) as well as giving you room (20%) to pay off debt and save adequately for retirement.

You might not be anywhere close to these guidelines, but they're something to shoot for as you get your finances in order. In the meantime, you can simply double your projected car payment and see how well that fits into your budget. If it doesn't, look for a cheaper car.

If you must borrow, stick to loans of 48 months or less. If the only way you can afford a car is with a longer loan -- or worse yet, by leasing -- then you really can't afford the car at all. A loan of four years or less will help keep you from overspending and allow you to build equity in the vehicle faster.

Own your cars longer. This one move can save you literally hundreds of thousands of dollars over your lifetime. Keep your cars for at least a year or two, if not longer, after you've paid them off. Set up an automatic savings plan so that the money that used to go for payments goes instead into a high-interest-rate savings account (ING Direct and EmigrantDirect.com are two options). Use the money to help pay for your next car. The longer you save, the less you'll have to finance.

Try to pay cash for your vehicles. In general, you don't want to borrow money to pay for assets, such as cars, that lose value, if you can possibly avoid it. You might not be able to pay cash for your next vehicle, but you certainly could for the one after that. Finance it for four years or less, keep it for an additional four years after you've paid it off, bank the payments and voilà: You have cash for your next car.

Liz Weston is the Web's most-read personal-finance writer. She is the author of several books, most recently "The 10 Commandments of Money: Survive and Thrive in the New Economy" (find it on Bing). Weston's award-winning columns appear every Monday and Thursday, exclusively on MSN Money. Join the conversation and send in your financial questions on Liz Weston's Facebook fan page.