(MONEY Magazine) – A holiday-inspired burst of generosity is a popular way to cut your taxes, since you can deduct gifts to charity as long as you itemize. But don't wait for year-end if you have assets other than cash or securities to donate. If you want to give a used car, for example, the charity may require paperwork that you might not be able to complete by Dec. 31, unless you get started soon. Even more important, the rules are tricky for determining how much you can deduct for nonfinancial contributions. To ensure the biggest possible write-off, you should take the time to consult a tax pro before you make a gift, especially if your donation is sizable.

Here's a tax primer on three common charitable gifts.

USED CARS You can deduct the fair market value of a used car that you give to charity. As a starting point for determining that figure, use the value given in the Kelley Blue Book used-car guide or the National Automobile Dealers Association's NADA Official Used Car Guide. But remember: Fair market value is what you could get for the car if you were to sell it on the date of donation. So if your car is a real clunker, you can't claim the Blue Book value as a deduction. Conversely, if you think your car is worth more than the used-car guides indicate, document your deduction by keeping a written record of the attributes your car had that the guides didn't account for. If you plan to deduct an amount that's much higher than average, get a mechanic's appraisal.

PRICEY ART, ANTIQUES AND COLLECTIBLES When you donate tangible personal property that has increased in value, the size of your write-off will depend on how long you have owned the asset and how the charity uses it. If you owned the item for more than a year before donating it, or if you inherited it from someone who had owned it for more than a year, you can deduct the fair market value--if the charity uses the item to further its stated tax-exempt purpose. Say you donate a painting you've owned for many years to a museum. The painting cost you $5,000 but is now worth $20,000. If the museum adds the painting to its collection, you can deduct $20,000. But if the museum sells the painting, your deduction is limited to $5,000, the price you paid for it--even if the museum buys another painting for its collection with the proceeds from selling yours. So to get a write-off for your gift's full fair market value, ask the charity for a letter stating that it intends to use your donation in a way that is directly related to the charity's mission, and be sure to keep the letter in your records.

If you donate an appreciated item that you've owned for a year or less, the tax law generally limits your deduction to the price you paid for the item, no matter how much it may have increased in value since you acquired it.

REAL ESTATE You can also write off the fair market value of land or buildings that you donate to charity, as long as you've owned the property for more than a year or inherited it from someone who owned it for more than a year. But real estate would usually result in either a capital loss or a capital gain if it were sold rather than given away--a fact that has important tax repercussions. For example, if you have a loss on your real estate, you're better off selling the property and giving the proceeds to charity. That way, you can deduct the loss on your tax return and claim a deduction for the charitable contribution. If selling your real estate would result in a long-term capital gain, however, you get a double tax benefit by simply giving it away because you avoid tax on the gain and you get a write-off for the property's fair market value. That's just the tax code's way of saying that giving doesn't have to hurt.