Smart tax planning is about more than knowing what to deduct. Just as important is knowing in which tax year to take deductions. That's right: Deciding to cram deductions into one tax year as opposed to another can have a direct bearing on the size of your tax bill.  Speeding up or postponing those write-offs by only a day at year-end can save you plenty of money.

For example, suppose you determine that you will pay less tax this year by claiming the standard deduction than by itemizing things like charitable con­tributions. Fine. But instead of losing these potential itemized deductions, you can delay them and trim taxes for next year. You could also do the reverse: Shift itemized deductions from next year to this year.

Simply put, you can "bunch" itemized deductibles into one of two years so they exceed your standard deduction. Then, for the other year, use the standard deduction if that is more advantageous than itemizing.

Even if you are never likely to use the standard deduction—perhaps because your state income taxes and property taxes always easily exceed it—you might profit from bunching certain deductions from one year into another. This is because some categories of deductions are only allowed when they exceed a certain proportion of your income.

Here are tips for moving deductibles into the year that works best for you:

MEDICAL EXPENSES. It's difficult, though not impossible, to deduct medical expenses. The big limitation is that the only outlays that are allowable are those you actually pay (i.e., expenses not covered by insurance or reimbursed by your employer), and then only for the portion that tops 7.5 percent of your adjusted gross income (AGI). You stand little chance of a tax break for medical costs unless your income is modest or you suffer the misfortune of unusually high medical expenses.

Say, for example, that you and your spouse have an AGI of $40,000 and unreimbursed medical costs of $5,000. The 7.5% floor cuts your deduction to just $2,000. So if you have big medical expenses, your goal should be to concentrate payments so that they top the 7.5% threshold in a given year.

PERSONAL INTEREST DEDUCTIONS. If you'll do better by itemizing and cramming deductions into a given year, consider avoiding interest charges on personal or "consumer" loans. There is no longer any deduction for interest on most of these loans. (There is a limited exception for student loans.)

Often a good way to pay off personal loans is with money from a home-equity loan. For couples, interest on home-equity loans of up to $100,000 ($50,000 for singles) is deductible.

STATE and Local TAXES. Another way to build up itemized deductions for this year is to pay both property taxes and the fourth-quarter installment of this year's estimated state and local income taxes before the year ends. This lowers your tax if you are going to be in a lower federal bracket next year, because the deductions are worth more to you this year.

If you think the amount now being withheld from your paycheck will not be enough to take care of your this year's state and local income taxes, you can ask your employer to withhold more from your remaining year's paychecks. Or you can make an estimated tax payment.

But heed this: Don't try to out smart the system by purposely overpaying your this year's taxes. The IRS says a payment of estimated state income taxes in December of the current year cannot be deducted for this year if the person making the payment did not reasonably expect to owe additional taxes at the time of the payment.

OTHER ITEMIZED DEDUCTIONS. Bunching miscellaneous, non medical expenses gives you a better shot at a deduction. Most of these expenses are deductible only to the extent that they, in aggregate, exceed 2% of your AGI. Your goal should be to speed up or postpone deductions into a year when you expect that they can surpass the 2% floor. If you are close to the threshold, maneuver into this year what would otherwise be next year write-offs—union dues, professional association dues, etc.