Getting a good deal on auto insurance and keeping your premium from rising is hard. Here are a dozen ways the industry works, with tips to help you save:

1. If you have good credit, you'll pay less. Almost all insurers pull your credit report. Studies show a direct correlation between your credit score and the likelihood that you'll file a claim. Insurers know that if you pay your bills on time and have the same credit accounts for a long time, you're more stable than someone who pays late and frequently opens/closes accounts. This information is used to create your "insurance risk score," a factor that determines your auto-insurance rate.

Tip: If you have unusual credit activity, wait a month for it to return to normal before buying auto insurance.

2. Your car model affects your premium. The auto insurers have a rating system for every car make and model. Most use a system devised by the Insurance Services Office, which starts with the vehicle cost, then factors in safety and theft data. Cars are rated from 1 to 27. Higher number means higher premium. If you're buying a new car, ask your insurer about the difference in premiums for cars you're considering. Search online for the latest top 10 lists on the most and least expensive cars to insure.

3. Pay in full to avoid installment fees. Payments usually are offered on a six-month, quarterly or monthly basis, but most insurers charge an administrative fee for breaking up the payments. The more you break it down, the more those fees add up.

Tip: Remember that insurance companies can cancel your policy for late payment, sometimes with minimal notification, so make sure you don't miss an installment. If you can pay the premium up front it may save you a few dollars.

4. That Beethoven CD in your car isn't covered. Stolen or damaged personal items aren't covered by your auto insurance.

Tip: You can file a claim on your home insurance. Most home-insurance policies will cover smaller, less expensive items such as compact discs. But if you carry expensive items such as computer equipment, ask about a rider to your home-insurance policy. It's wise to take photos or video of any expensive personal items before they go missing.

5. You'll pay for your bad driving. The industry standard is to increase your premium by 40% of the insurer's base rate after your first at-fault accident. For example, if the company's base rate is $400, your premium will go up by $160. Not all auto insurers play by this rule, though, and some may increase your individual rate by 40%. Regardless of what formula they use, most of the time, your rates will increase.

Tip: Some insurance companies have a "forgive the first accident" policy. The qualifying variables are wide-ranging, so ask your company if it has a forgiveness policy and how to qualify.

6. You'll pay for your friend's bad driving, too. If your friend borrows your car and crashes it, you'll have to file a claim with your insurance company. You'll have to pay any deductible that applies, and your rates will probably go up as a result of your claim.

Tip: If your friend didn't have permission to take your car, in most cases you won't be held liable for the damage. But if your friend is uninsured and causes damage that exceeds your policy limits, the injured party can come after you for medical and property-damage expenses. Best bet? Don't lend out your car.

7. Your car's real worth. The value of your "totaled" car may surprise you. Auto-insurance companies don't use the standard Kelley Blue Book or National Association of Automobile Dealers value. Instead, each company has its own proprietary list of car values, and most have specialized software for valuing cars in each region. They take into consideration the car's mileage and pre-accident condition. The insurance company may also ask local dealers what they'd charge for a similar replacement car. However, the insurer will consider quotes from suburban towns as reasonable estimates. You might have to drive several hours to reach the cheapest dealer, just to save the insurance company money. And they might be quoted a better deal than you could get if you walked onto the lot.

Tip: If you disagree with your insurance company's value determination, there are several things you can do:

Next time, get "gap" insurance. It will pay the difference between what an insurer will cover and what you owe, which can be several thousand dollars.

If you have maintenance records that show you've had the oil changed every 3,000 miles and you've had the car checked routinely by a mechanic, present copies to the insurance company to show the car was maintained. If you've been paying premiums on any special parts or upgrades, make sure those are included in the insurance company's evaluation.

Get price quotes on replacement cars from three dealers within a reasonable driving distance and submit these to your insurance company. Ask the insurance company for a list of dealers within a specific distance who can sell you an equivalent car for the value the company is claiming. If you still aren't satisfied, you can step up the process and go to mediation or arbitration. Mediation involves presenting your case to a neutral party for help in reaching a compromise; arbitration is a binding decision. You can also, of course, take the issue to court.

8. Check into "diminished value." Say your car has been in an accident, but repaired. Is it worth less than the exact same car that hasn't been in an accident? It's a hot topic, but some say yes. In 14 states, you're allowed to file a claim with your insurance company for that lost value.

Tip: Thirty-six states and Washington, D.C., allow insurers to exclude payments for diminished value, so if you live in one of those states, you can't claim the loss. But in Florida, Georgia, Hawaii, Kansas, Louisiana, Maine, Maryland, Massachusetts, North Carolina, South Dakota, Texas, Virginia, Washington and West Virginia, you have a chance of getting a diminished-value payment. If you weren't at fault in the accident, you often can make a successful case against the insurance company of the at-fault driver.

9. You may not owe sales tax on your replacement car. Twenty-eight states require auto insurers to pay for the sales tax when you replace your totaled vehicle with a new or used car: Alaska, Arizona, Arkansas, California, Connecticut, Florida, Georgia, Hawaii, Illinois, Indiana, Kansas, Kentucky, Maryland, Minnesota, Missouri, Nebraska, Nevada, New Jersey, New York, North Dakota, Ohio, Oklahoma, Oregon, South Dakota, Vermont, Washington, West Virginia and Wisconsin.

Tip: Make the request; don't expect the insurer to offer to pay upfront. Even in states that do not require sales-tax reimbursement, you should request it. Many auto insurers will not deny the request because the policy requires that they make you "whole," returning you to where you were before the accident at no cost to you.

10. The tax will be calculated based on the pre-accident value of your car. If the insurance company values your car at $10,000, and you purchase a new car for $20,000, the tax will be calculated on $10,000.

11. You can wait to add your teenager to your policy until he or she is licensed. You're not required to add your teenager to your policy just because he/she has reached driving age. Usually you can wait until he/she has a license or, if you're in a high-risk insurance pool, a permit.

Tip: Don't forget to tell your insurance company that you have a licensed teen. If you have to file a claim on his/her behalf, your insurer is entitled to charge you back premiums from the date your teen received a license.

12. You must officially cancel your insurance policy when you switch insurers. Your policy likely states that you can cancel by notifying the company in writing of the date of termination. Don't assume that you can terminate the policy at the end of the coverage period by simply ignoring the bill. The insurers won't see it that way. They'll send you another bill for the next premium payment, and when you don't pay it, you'll be cancelled for nonpayment. That goes on your credit record.

Tip: Call your insurance agent or company and state that you're canceling your policy. Give a specific date, or you may end up uninsured for a period of time. The company will send you a cancellation request. Often, the form is already filled out and just requires your signature. Read it to check for errors. You may have to provide proof of new coverage to your former insurer. And if you've financed through a dealership, give the dealer your new insurance information, because purchase contracts often require proof of coverage.