Note especially the advice for two earner families as many of my clients have found at the end of the year if they used the withholding for one earner when they both worked, that at the end of the year, they had not withheld enough and had to pay 

IRS issues 2012 Form W-4:  The IRS has released the 2012 version of Form W-4, Employee's Withholding Allowance Certificate. The instructions note that an employee cannot claim exemption from withholding if: (1) the employee's income exceeds $950 (unchanged from 2009-2011) and includes more than $300 of unearned income (for example, interest and dividends), and (2) another person can claim the employee as a dependent on his or her tax return. Employers must withhold from the pay of any employee who had claimed exemption from withholding in 2011, but who does not provide a new Form W-4 to continue the exemption by Feb. 15, 2012.

The dollar amounts used in the computation of allowances for employees eligible for the child tax credit will remain the same for 2012. Employees whose total income will be less than $61,000 ($90,000 if married), should enter “2” allowances on the personal allowances worksheet for each eligible child; then less “1” if they have three to seven eligible children, or less “2” if they have eight or more eligible children. See line G of the Personal Allowances Worksheet on page 1 of Form W-4 for further information.

The IRS advises employees who have more than one job, or who are married and both spouses work, with combined earnings from all jobs exceeding $40,000 ($10,000 if married), to look at the Two-Earners/Multiple Jobs Worksheet on page 2 of the instructions to avoid having too little tax withheld. The IRS also advises employees to use the 2012 version of IRS Publication 505, Tax Withholding and Estimated Tax, after Form W-4 takes effect, to confirm that the amount they are having withheld is sufficient, particularly if the employee's income is projected to exceed $130,000 (single) or $180,000 (married) in 2012