A great thing about a 401(k) is that you can sign up and forget about it. The money you set aside disappears from your paycheck before you get your hands on it.

Although this autopilot approach is relatively painless, don't be lulled into complacency. Keep your eyes open for ways to slip more cash into this tax shelter. If you're not maxed out already, don't overlook these opportunities:

• Maxed out on social security taxes? This 6.2% levy stops when earnings pass $97,500 in 2007. If you cross the threshold, boost your contributions so your 401(k) gets the cash the government used to take.
  
  
• Refinanced your mortgage? Sock some of your savings from the lower rate into your 401(k).
  
  
• Refinanced your student loans? If you have student loans, check to see if you can consolidate them at a lower rate.
  
  
• End-of-year bonus? Ask if you can deposit part of it in your 401(k). Rules vary.
  
  
• Are you overwithholding? If you got a tax refund this spring, file a new W-4 form with your employer to cut your withholding. Use our calculator to figure out how much you can add to your paycheck. At the same time, boost your 401(k) contribution by the same amount. Your paycheck won't change, but your retirement security sure will.

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