If you're like most people, you probably pay Uncle Sam throughout the year by having your employer withhold tax from your paychecks.

Your employer, using tables supplied by the government, determines how much of your paycheck should be withheld based on information you provide.

Surprised? That's because you've probably forgotten about that Form W-4 you filled out, something most people do when they start a new job. The W-4, which can be amended at any time, lets you mark your tax filing status (single, married, etc.) and the number of allowances you want to take. An allowance essentially reduces the amount of taxes withheld, and increases the amount of your take-home pay. Each allowance represents an exemption, credit or some other tax benefit you plan to claim when you fill out your return. (For detailed instructions on adjusting your tax withholding, see IRS Publication 919.)

Your goal at the beginning of every tax year should be to withhold at least 90 percent of what you think you'll owe for that year -- but not much more. "If you use the worksheet that accompanies your W-4, you should definitely have that 90 percent covered," says Tony Bardi, an enrolled agent in Gresham, Ore.

Each January, your employer sends you and the IRS a Form W-2 that reports your earnings for the prior tax year and the total amount of tax you had withheld. You're then responsible for calculating how much more you owe (and paying the difference by April 15), or, figuring out how much the IRS should refund you if you overpaid.

Although a lot of people consider a refund found money, the truth is, getting a refund check just means you've given the government an interest-free loan. It's money you earned and should have had access to throughout the year. Say you get a $1,200 refund (the average is about $1,700). You could have pocketed more money if you had adjusted your withholding so that you got an extra $100 a month and invested that money in an interest-bearing account. Or, if you carried a credit card balance, the extra amount could have been used to pay off more of your high-interest debt.