Make sure to stay on top of the deadlines for filing federal tax returns and the due dates for making payments. Miss just one, and the IRS might exact a sizable, nondeductible penalty.

Friday, Jan. 15, 2010, is a key date for many taxpayers to remember. For individuals whose estimated income tax exceeds $1,000, that's the due date for the final quarterly installment of estimated income tax for 2009 — including any self-employment tax and alternative minimum tax. But it's okay to skip this final payment, provided they submit their 2008 returns and pay their tax in full by Monday, Feb. 1.

Who needs to make estimated payments? Individuals with income from sources not subject to withholding of taxes. This category mostly comprises self-employed individuals who operate businesses or professions as sole proprietorships, in partnerships with others, or as independent contractors, and investors who receive sizable amounts of interest, dividends and profits from sales of assets.

To avoid unnecessary payments, remember to take account of withholding during 2009 on what you or your spouse receive from salaries, wages and other types of compensation. Ditto for an overpayment of taxes in 2008 that you elect to apply to your 2009 bill.

The IRS can exact penalties for failing to pay sufficient tax during the year through withholding or quarterly payments, as well as for failure to pay required installments on time as they become due. It matters not that your final estimated payments are sufficient to erase any balance due when you submit your 2009 1040 form in 2010.

However, there are "safe harbors" or exceptions that excuse you from any penalties for underpayments of more than $1,000 for withheld or estimated taxes. For relief from these penalties, you must satisfy a two-step requirement:

First, make payments by the quarterly due dates — for 2009, by April 15, June 16, Sept. 15, and Jan. 15.

Second, those payments must at least equal any of the following three amounts:

(1) 90 percent of the total tax for 2009 (reduced to 66 2/3 percent for qualifying farmers and fishermen).

(2) 100 percent of the total tax for 2008. This is the amount on line 61 of the 2008 1040 form.

The exception based on the prior year's tax is available even if the amount due was zero, provided the return covered 12 months, as it ordinarily would.

As the prior-year exception uses a fixed number, it's the easiest way for most individuals to figure their payments and avoid penalties. To illustrate, your tax payments total $12,000 for 2008 and $15,000 through estimates or withholding in 2009. With those kinds of numbers, you're home free, no matter how much extra you owe when you file for 2009.

There's a restriction on use of this exception when adjusted gross income for 2008 (the amount on the last line of page one of Form 1040) exceeds $150,000 — declining to $75,000 for married couples who file separate returns. To take advantage of the 100-percent escape hatch, payments must equal 90 percent of the total tax for 2009 or 110 percent of the total tax for 2008 — whichever is less.

Small business owners qualify for a break if their 2008 AGI was less than $500,000 ($250,000 if married and filing separately) and more than half of their gross income was from a firm with less than 500 workers. They can base payments on the lesser of 90 percent of 2008's tax, rather than the 100 or 110 percent benchmarks.

(3) 90 percent of the total tax for 2009, figured by "annualizing" income actually received by the end of the quarter in question.

The annualizing exception helps those whose incomes unexpectedly increase or fluctuate throughout the year, such as Roth IRA converters who move money out of traditional IRAs and into Roth accounts at year's end, or investors who receive higher than anticipated distributions of dividends and capital gains from their taxable accounts with mutual funds. But be warned: this calculation is complicated.

Julian Block is an attorney and author based in Larchmont, N.Y. He has been cited as "a leading tax professional" (New York Times), "an accomplished writer on taxes" (Wall Street Journal) and "an authority on tax planning" (Financial Planning Magazine). His books include "The Home Seller's Guide To Tax Savings." Law professor James Edward Maule of Villanova University praised the book as "An easy-to-read and well-organized explanation of the tax rules. Home sellers would be well advised to buy this book." To order it, go to www.julianblocktaxexpert.com.