The trick is the higher the number of allowances, the less they take out for taxes. If you mark single (or married, but withhold at higher single rate) they'll take out more than if you mark just plain married. If both you and your spouse work, I've heard it's best to mark the married but withhold at higher single rate. The plain married one is mostly for 1-income couples, as I understand it.http://www.irs.gov/pub/irs-pdf/fw4.pdf
On top of that, you can request that your employer withhold an additional amount. (Line 6) If you work through the worksheet on the W-4 (which can be confusing!) it should help you - but your tax person should be able to tell you how you should change your withholding.
By the way, you can change your withholding at any time - you aren't locked in to what you requested when you first started your job.