People whose taxes are not withheld in regular wage checks must make estimated tax payments. This applies to business owners and the self-employed. The IRS assesses penalties for underpayment of taxes throughout the year. You should be safe if you're paying 90% of your expected tax bill or 100% of last year's tax bill (110% if your adjusted gross income is above $150k). Your tax professional should provide you with quarterly payment slips when you file each year's return. If you can, increase withholding in December if you are facing a potential underpayment penalty.

Small business owners have a break on 2009 estimated tax rules. If your 2008 adjusted gross income was under $500k and more than half of it was from a firm with under 500 workers, your payin requirement is the lower of 90% of your 2008 or 2009 tax liability. This could free up some cash this year-end (actual taxes are still due on April 15th).

Estimated taxes and underpayment penalties can be tricky, if you're in doubt, its worth a call to your tax professional. If you don't have a tax professional, dealing with these issues may be a good reason to get one.