No one's perfect -- certainly not the Internal Revenue Service. It does, after all, report more than four million math errors a year.

You have the right to dispute any bill you think is wrong.

Two of the most common IRS errors are billing you for interest and penalties on a bill you've already paid and challenging a legitimate, documentable deduction.

If your taxes were prepared by a certified public accountant, ask your CPA to call the IRS. He or she is responsible for any errors and is most knowledgeable about your tax situation.

If your taxes were prepared by one of those giant tax preparers like H&R Block, your contract will spell out how much help you can expect.

H&R Block, for example, says it will answer "questions regarding the preparation of your return," but you'll have to deal with the IRS. Block also will cover and any penalties or fees you're assessed -- if you can prove Block made a mistake.

Customers who paid an extra $30 for Block's "Peace of Mind" plan can ask a company representative to contact the IRS on their behalf and expect that the firm will pay up to $5,000 in additional taxes if it accepts responsibility for an error.

But we've heard so many horror stories about H&R Block, Jackson Hewitt and other tax preparers that we wouldn't be surprised if you have a difficult time obtaining all of the help you expect or getting them to own up to mistakes.

You'll probably have to deal with the IRS pretty much as if you'd prepared your own taxes.

If you did your own taxes, you have four ways to dispute a tax bill:

• Call the number on your bill.
• Go to the IRS' Web page on appealing disputes.

• Visit your local tax office.
• Write to the address on your bill.

Visiting your local IRS office is the quickest way to get an answer.

Yes, you might have to take a day off work, and, yes, it might be tedious. But by going in person, you'll know whom you're talking to and you're physically there to deal with an issue. It's a lot easier to ignore a piece of paper than a person, so take the time to straighten out your issue.

You'll usually need to bring:

• Your tax return.
• Your tax bill.
• All records pertaining to that bill. That could be receipts or other documents supporting a disputed deduction, or canceled checks that show when a previous bill was paid.

Make copies of everything for the IRS representative. Keep the originals.

Above all, keep the conversation focused on the bill in dispute. Don't discuss other issues or present the IRS representative with any other documents.

"You should never take anything other than exactly what they ask for," says Kenneth E. Guard, a certified financial planner in Ft. Myers, Fla. "The more you take, the more they'll look at, and the more likelihood they'll find something where you made a mistake."

If you're frustrated with the process, or think that you're right even if the IRS tells you you're wrong, the next step is to hire a CPA or tax lawyer.

If that is out of your price range, you have two options. The first is the Low Income Taxpayer Clinic, a free service that can help you work with the IRS in everything from audits to appeals and collection disputes.

The second is the IRS' Taxpayer Advocacy Service. It's an independent group within the IRS created to work with you to sort through your debt and figure out the best way to pay it.

You also can use this service if you can't afford independent counsel, or if you've filed a request with the IRS and haven't heard back within 30 days or by the date the IRS told you it would reach a decision. Every state, plus the District of Columbia and Puerto Rico, has at least one taxpayer advocate.

By Jen A. Miller

Interest.com Contributing Editor

Have questions about your finances? Ask us at editors@interest.com.