It happens every year. In their haste to get their tax returns filed on time, millions of taxpayers make mistakes that can affect their tax bills, delay the processing of their returns, or draw the attention of the Internal Revenue Service (IRS). So before you seal that envelope or push the key that will send that return into the great IRS beyond, beware of some of the most common errors:

• Make certain you have completed and attached to your return all required schedules and forms. Attach them behind your Form 1040 using the "Attachment Sequence Number" shown in the upper right corner of each schedule or form. All other statements and attachments should go at the end of your return, even if they relate to another form or schedule. It's a good idea to include your name and Social Security number on each page of every form you send the IRS.

• Be sure your return includes the Social Security number for all dependents who were born on or before November 30, 1996. Failing to do so will subject you to loss of the exemption. Also, if you are claiming a tax credit for a child or other dependent, you need to provide the caregiver's name, address, and taxpayer identification number or Social Security number.

• Review your return and schedules to make sure that your math is correct. If you find an error and need to change a figure, determine if the change affects other figures on your return and be sure to change them as well. Better yet, consider using a tax return preparation computer program that checks the numbers for you.

• Document all charitable donations. Remember, a canceled check is no longer sufficient to support a donation of $250 or more. You must have a written acknowledgment from the charity. If your gift was one of property rather than cash, the acknowledgment must describe the property. When your noncash charitable contribution exceeds $500, you also are required to file IRS Form 8283, Noncash Charitable Contributions, giving details of the donation.

• Review last year's return to see if there are any items to be carried forward to this year, such as capital losses or charitable deductions that exceeded the amount you were able to deduct in previous years. The review will also remind you of items of income that should be reported or expenses deducted.

• Check to see that you have reported investment income in the right place. Some taxpayers mistakenly report earnings from money market funds as "interest income" when, in fact, the IRS considers such earnings to be dividends.

• Do not declare your state tax refund as income on your return if you did not receive a tax benefit from deducting the taxes. Many people make the mistake of automatically reporting state tax refunds as income on their federal return. Rather, if you took the standard deduction instead of itemizing in 1996, you don't have to show your state tax refund as income for 1997.

• Make sure you have dated and signed your return as well as your check if taxes are due. If you are filing a joint return, your spouse must sign the return as well, even if he or she did not report any income. Make your check out to the IRS and be sure the front of the check shows your name, address, Social Security number, form number, and tax year. Do not attach your check to the form. And, to help speed the processing of your return, use the mailing label and bar-coded envelope that comes in your tax package.

• Make copies of everything you send and consider filing your return by certified mail with return receipt requested. If your return is lost in the mail, the tax code recognizes a certified or registered mail receipt as proof of filing.