For millions of Americans, the most important tax-planning tip between now and the end of the year is to check your charitable contributions.

They are on Schedule A, Line 19 of your tax return. Do these donations, plus your mortgage interest, allowable medical expenses, and $10,000 of state and local taxes, normally add up to more than $12,000 if you are single and $24,000 if you are married filing jointly?

Unless...