Talk about a spring awakening: If you haven't been paying close attention to receipts and files since the last time tax day rolled around, you've now got until mid-April to wrestle your taxes into shape. You don't want to pay more than you need to, of course. Yet there are more than 350 possible deductions on your federal return alone, says TurboTax CPA Lisa Greene-Lewis, and even professionals don't always take the right ones. A 2014 Government Accountability Office study tested 19 randomly selected tax preparers and found that only two managed to calculate the correct refund.
Money asked experts to identify some of the areas that cause the most trouble. Make sure you're not stumbling into any of these traps.
1. Missing a sales tax upgrade
Late last year, Congress made the sales tax deduction permanent, which is good news for residents of states with no or nearly no state income tax—Alaska, Florida, Nevada, New Hampshire, South Dakota, Tennessee, Texas, Washington, and Wyoming. (You can deduct sales tax or state income tax but not both.) You can use a formula on your federal form to take a deduction for sales tax paid; you don't even need receipts. But you can increase your deduction by adding the sales tax you paid on major purchases: Car sales, for instance, hit a record in 2015, and rack up a big tax bill (see chart at right). Hefty outlays for home improvements can tip the scales as well, says Dave Stolz, a CPA in Tacoma.
2. Overlooking investment losses
Last year's volatile market left some investors underwater. If you sold at a loss during August's slide, you can deduct up to $3,000 from ordinary income after you've offset capital gains. You can carry forward any additional losses to offset income in future tax years.
3. Ignoring medical expenses
Medical costs are deductible on your federal return, but only to the extent they exceed 10% of your adjusted gross income, or AGI (7.5% if you or your spouse is at least 65). If you had big out-of-pocket expenses last year, add up your bills: You can deduct nearly any out-of-pocket medical or dental cost—drugs and doctor bills but also mileage traveled to appointments, disability-related home or car modifications, addiction treatment, lead paint removal, a guide dog. (Jean-Luc Bourdon, a CPA in Santa Barbara, remembers a client who successfully deducted clarinet lessons prescribed to treat a speech impediment.)
Read Next: 30 Tips for Doing Your Own Taxes
In the ballpark but not quite over the plate? Try to reduce your adjusted gross income with an extra IRA or HSA contribution. And check to see if your state has a lower threshold that lets you deduct medical expenses on your state return; several do, including Arizona and New Jersey.
4. Taking the wrong college write-off
Paying a dependent's college costs? You can benefit from the American Opportunity Tax Credit of up to $2,500, the $2,000 lifetime learning credit, or a $4,000 tuition and fees deduction—but you can't take them all in the same year. Tax credits tend to be worth more than deductions, because they directly reduce the amount you pay, so the AOTC may be the way to go, if your income qualifies. You can also take the credit per student, not per return, so it's especially valuable if you had more than one kid in college.
Run your scenario with each of the three choices and pick the one that benefits you most. Keep in mind that the deduction would reduce your family's AGI, which might get you more need-based aid this fall.
5. Not adding up child care
If you need a babysitter in order to work, look for work, or attend school, take a tax credit that's worth 20% to 35% (depending on your income) of up to $3,000 in expenses for one child or up to $6,000 for two or more. The cost of day camp counts, though not sleepaway camp. If you've set aside pretax dollars in an FSA for child care, you must use that money first, but you can still take a credit for additional expenses up to the limit once you've exhausted the account. There's no income restriction, but the benefit disappears when Junior turns 13. Also, your caregiver must have a Social Security number or tax ID.
6. Picking the wrong work-related expenses
Unless you're self-employed, there are few legitimate job-related deductions -- though a lot of people try to stretch the definition, say accountants. One likely trouble spot: Work clothing doesn't count, unless it's a uniform or other specialized gear. "If you can use it outside work, you can't deduct it," says Cari Weston, taxation director for the American Institute of Certified Public Accountants. "The Armani suit is not deductible. The race-car suit is." Commuting costs are not deductible -- but miles you drive on the job are fair game, if you don't get reimbursed (and only to the extent they and other miscellaneous deductions top 2% of AGI). Same goes for miles you drive to look for a new job. Teachers can take a $250 deduction for classroom supplies -- a write-off that Congress just made permanent.
7. Deducting your kids' student loan interest
Paying off student loans? If they're yours, great; you can deduct the interest even if you don't itemize, as long as your AGI is no more than $80,000 (if single) or $160,000 (married filing jointly). But parents beware: If you're making your kid's loan payments, that's a gift, and the interest isn't deductible.
8. Failing to document charitable expenses
You can write off not only checks written and goods donated, but also supplies for charitable work. Equipment purchased and mileage driven are deductible (although the value of your time and the services you donate are not). But keep careful records, and get acknowledgment from the charity for any gift worth $250 or more. "You need the receipt before you file your return, because the IRS won't let you go back and ask for a receipt once an audit begins," says Stephen R. Allen, a CPA in Lexington, Ky.
9. Missing self-employment deductions
Self-employed people can claim a long list of deductions, including business expenses, retirement plan contributions, 50% of self-employment tax, and workspace. People are afraid that claiming a home office will trigger an audit, says Jode Beauvais, a CPA in Tacoma. "If you follow the rules, there is no reason to be afraid to put it on your tax return."
Read next: 9 Signs of a Bad Tax Preparer
The same goes for car usage, she says. "Keep accurate records. If you do that, there is nothing to be afraid of." Health insurance premiums for you and your spouse and dependents are also deductible if you're self-employed.
10. Not revisiting past returns
Same-sex married couples in states that didn't recognize those unions before the Supreme Court legalized gay marriage nationwide may find their state tax burden has actually increased, says Bourdon. (Thank you, marriage penalty.) But if your newly recognized marriage does cut your state tax, check to see how far back you can amend past years' state returns.
11. Letting a loser Roth conversion stand
If you owe big taxes on a 2015 Roth conversion that's lost significant value, you can get a do-over by recharacterizing it before Oct. 17, 2016. "Let's say that you converted an IRA to a Roth when the account had assets worth $50,000," says Troy Lewis, a CPA in Draper, Utah. "In the 25% tax bracket, you would owe $12,500 in taxes." But if market turmoil has slashed those assets to just $40,000, you can transfer them back to your IRA to undo the move. Then, if you've already filed for 2015, put in an amended return to get back the tax you paid on those gains.
Once the assets are back in an IRA, you can reconvert them into a Roth after 30 days if that's still to your advantage. "This gives you a chance to reset the value and convert it again," Lewis says.
12. Overlooking health plan paperwork
If you bought insurance on an Obamacare marketplace, you'll get a new form this year. You don't need to file the 1095-A, but don't overlook it: You'll use it to complete your Form 8962, which will help you make sure you're taking all the credits you're allowed -- if, for example, your income dropped after you first signed up -- and to see whether you owe any penalties for lack of coverage.
One more filing note: The tax deadline is April 18 this year, because of a Washington, D.C., holiday. In Massachusetts and Maine, because of Patriots' Day, the deadline is April 19.