Every year, the IRS dutifully reports the most common blunders taxpayers make on their returns. And every year, at or near the top of the list, is forgetting to enter a Social Security number or making a mistake when entering the nine digits that identify us to IRS computers.

Before you bemoan such foolishness, ask yourself a simple question: Is that the most common error, or just the most easily noticed goof?

Who knows how many people forgot—or never knew about—a deduction that could save them money? That’s not the kind of thing over which government bean counters lose a lot of sleep.

No doubt about it: The opportunity for mistakes is almost unlimited. The most recent numbers show that about 46 million of us itemized deductions on our 1040s—claiming nearly 1 trillion dollars’ worth of deductions. That’s right: $1,000,000,000,000! Another 85 million taxpayers claimed more than half a trillion dollars’ worth of standard deductions. Some of those who took the easy way out probably shortchanged themselves. (If you turned 65 in 2008, remember that you deserve a bigger standard deduction than younger folks.)

Years ago, the head of the IRS told Kiplinger’s Personal Finance magazine that he figured millions of taxpayers overpaid their taxes every year by overlooking just one of the money-savers listed below. Without further ado, here are our 11 most overlooked tax deductions. Claim them if you deserve them, and keep more money in your pocket.

This write-off makes sense primarily for those who live in states that do not impose an income tax. You must choose between deducting state and local income taxes, or state and local sales taxes. For most citizens of income-tax states, the income tax deduction usually is a better deal. IRS has tables for residents of states with sales taxes showing how much they can deduct. But the tables aren’t the last word.

If you purchased a vehicle, boat or airplane, you get to add the state sales tax you paid to the amount shown in IRS tables for your state, to the extent the sales tax rate you paid doesn’t exceed the state’s general sales tax rate. The same goes for home building materials you purchased. These items are easy to overlook. The IRS even has a calculator on its Web site to help you figure out the deduction, which varies by your state and income level.

This isn’t really a deduction, but it is a subtraction that can save you a lot of money. This is the break former IRS Commissioner Fred Goldberg told Kiplinger’s that lots of taxpayers miss. If, like most investors, you have mutual fund dividends automatically invested in extra shares, remember that each reinvestment increases your “tax basis” in the fund. That, in turn, reduces the taxable capital gain (or increases the tax-saving loss) when you redeem shares.

Forgetting to include the reinvested dividends in your basis—which you subtract from the proceeds of sale to pinpoint your gain—means overpaying your tax. TurboTax Premier and Home & Business tax preparation solutions include a very cool tool—Cost Basis Lookup—that will figure your basis for you and make sure you get credit for every dime of reinvested dividends.

It’s hard to overlook the big charitable gifts you made during the year by check or payroll deduction. But the little things add up, too, and you can write off out-of-pocket costs you incur while doing good deeds. Ingredients for casseroles you regularly prepare for a nonprofit organization’s soup kitchen, for example, or the cost of stamps you buy for your school’s fundraiser count as a charitable contribution. If you drove your car for charity in 2008, remember to deduct 14 cents per mile (if driving to aid victims of the floods and tornadoes in the Midwest, you can deduct 35 cents per mile for driving during the first half of the year, and 41 cents per mile for driving during the last six months).

Until recently, if parents paid back a student loan incurred by their children, no one got a tax break. To get a deduction, the law held that you had to be both liable for the debt and actually pay it yourself. But now there’s an exception. If Mom and Dad pay back the loan, the IRS treats it as though they gave the money to their child, who then paid the debt. So a child who’s not claimed as a dependent can qualify to deduct up to $2,500 of student loan interest paid by Mom and Dad.

Here’s an interesting dichotomy: Job-hunting expenses incurred while looking for your first job are not deductible, but moving expenses to get to that first job are. And you get this write-off even if you don’t itemize. If you moved more than 50 miles, you can deduct the cost of getting yourself and your household goods to the new area, including 19 cents per mile for moves during the first six months of 2008, and 27 cents per mile for job move-related driving after June 30 (plus parking fees and tolls) for driving your own vehicle.

If you are a member of the National Guard or military reserve, you may earn a deduction for travel expenses to drills or meetings. To qualify, you must travel more than 100 miles and be away from home overnight. If you qualify, you can deduct the cost of lodging, half the cost of your meals, 50.5 cents per mile for qualifying driving during the first six months of the year and 58.5 cents per mile for qualifying driving after June 30, plus any parking or toll fees for driving your own car. You get this deduction whether or not you itemize.

A credit is so much better than a deduction—it reduces your tax bill dollar for dollar. So missing one is even more painful than missing a deduction that simply reduces the amount of income that’s subject to tax.

But it’s easy to overlook the child care credit if you pay your child care bills thorough a reimbursement account at work. Until a few years ago, the child care credit applied to no more than $4,800 of qualifying expenses. The law allows you to run up to $5,000 of such expenses through a tax-favored reimbursement account at work.

Now, however, up to $6,000 can qualify for the credit, but the old $5,000 limit still applies to reimbursement accounts. So if you run the maximum $5,000 through a plan at work but spend more for work-related child care, you can claim the credit on up to an extra $1,000. That would cut your tax bill by at least $200.

This sounds complicated, but it can save you a lot of money if you inherited an IRA from someone whose estate was big enough to be subject to the federal estate tax. Basically, you get an income tax deduction for the amount of estate tax paid on the IRA balance.

Let’s say you inherited a $100,000 IRA and the fact that the $100,000 was included in your benefactor’s estate added $45,000 to the estate tax bill. As you withdraw the money from the IRA and pay tax on it, you also get to deduct a proportional amount of the estate tax paid. If you withdraw $50,000 in one year, for example, you get to claim a $22,500 itemized deduction on Schedule A.

Did you owe tax when you filed your 2007 state tax return in the spring of 2008? Then remember to include that amount with your state tax deduction on your 2008 return, along with state income taxes withheld from your paychecks or paid via quarterly estimated payments.

When you buy a house, you get to deduct points paid to obtain your mortgage in one fell swoop. When you refinance a mortgage, however, you have to deduct the points over the life of the loan. That means you can deduct 1/30th of the points a year if it’s a 30-year mortgage—that’s $33 a year for each $1,000 of points you paid. Doesn't seem like much, but why throw it away?

Also, in the year you pay off the loan—because you sell the house or refinance again—you get to deduct all the points not yet deducted, unless you refinance with the same lender. In that case, you add the points paid on the latest deal to the leftovers from the previous refinancing and deduct the expense, which is pro-rated over the life of the new loan.

Some employers continue to pay employees’ full salary while they are doing their civic duty, but ask that they turn over their jury fees to the company coffers The only problem is that the IRS demands that you report those fees as taxable income. You’ve always had a right to deduct the amount so you weren’t taxed on money that simply passed through your hands.