Missed breaks are costing you more in taxes

From start to finish, the average taxpayer filing a Form 1040 spends 16 hours on his or her taxes—including eight hours combing through records. Yet despite that scrutiny, you could still be leaving money on the table.

"Sometimes folks just automatically assume their taxes are going to be the same as they were in the prior year," said Melissa Labant, director of tax advocacy for the American Institute of Certified Public Accountants. "That is dangerous." Changes in tax law, changes in your financial situation or a combination of both can mean you're eligible for a host of breaks that you weren't before.

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"The one I see that gets overlooked a lot is charitable contributions," said Labant. Consumers usually comb through their checkbook and credit card statements, but can miss cash donations and donations of physical goods. Some companies let workers donate from their paycheck, but that won't show up on your W-2, she said—you'll need to look at your year-end paystub for the total.

And volunteers might not realize they can deduct mileage driven to and from charitable activities, at 14 cents per mile.

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The retirement savers credit is a bigger-ticket benefit that often gets passed over. "That's a tax break of up to $1,000, or up to $2,000 if you're married, for making contributions to retirement accounts," said Labant. Taxpayers just starting out don't often know about it, and so miss out. Others don't even consider the break due to its low income threshold—single filers making more than $30,500 and married filing jointly making more than $61,000 don't qualify. But if you were unemployed or retired for part of the year, or had a bad year as a freelancer, your income might have dipped enough to grab at least a partial credit, she said.

Another possible miss? The dependent care credit, which is worth as much as 35 percent of up to $3,000 in qualifying expenses for one child (or other qualifying person); $6,000 for two or more. Most parents know to grab the break for daycare and nanny services, but the IRS also allows some qualifying medical expenses and day camps.

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Even after April 15, make sure you're not missing out on other valuable tax breaks for the rest of 2015—namely, pre-tax contributions to an employer-sponsored retirement plan or flexible spending accounts for medical, child care and transit expenses. "That's money left on the table," Labant said.