One of the most difficult approaches for many taxpayers and tax preparers is to think of tax liability in two-year chunks.

From now until Dec. 31 you should be considering tax years 1982 and 1983 as a correlated unit. Then starting Jan. 1 you want to consider the tax impact of financial transactions on 1983 and 1984 in tandem.

Perhaps the one tax-reduction technique that most clearly demonstrates the importance of dual-year consideration is alternating deductions.

This is most applicable to taxpayers who generally find that their total deductions each year hang right around the zero-bracket amount (standard deduction): $2,300 for a single person, $3,400 on a joint return.

"Alternating" simply means bunching as many deductible expenses as you can into one tax year, and itemizing on Schedule A for that year. Then the following year you keep your tax-deductible expenses as low as possible, then use the zero bracket amount (ZBA).

Suppose you file a joint return, and when you added up all your deductions for 1981 you ended up with $3,100. So you threw them all away and took the $3,400 ZBA. Now you repeat the exercise for 1982 and reach $3,200 -- so again it's the ZBA route.

That means that for the two years you spent $6,300 on items that could have qualified for itemizing. But you wound up with the ZBA each year for a two-year total of $6,800.

Now suppose that you looked at 1981-82 as a package late last year, and were able to slip $1,000 of your potential deductions from last year to this year.

Maybe you postponed the year-end payment on a church pledge for a couple of weeks; or let a car loan payment go from Dec. 28 to Jan. 1; or paid your health insurance premium, due Jan. 1, on that date instead of a week earlier as you usually do.

Let's see what that does to your tax liability. For 1981 you now add up $2,100 in deductible items, so you do not itemize and instead use the $3,400 ZBA. But look at 1982: With this $1,000 you shifted you now have a total of $4,200 -- so you itemize.

You now have a ZBA of $3,400 in 1981 and Schedule A deductions of $4,200 in 1982 -- a total for the two-year period of $7,500 contrasted with the $6,800 using the ZBA both years.

If you're in the 31 percent tax bracket, you're looking at a $248 tax saving for the two years (plus a little more on your state tax) -- achieved with the same $6,300 in expenses, simply by adjusting the timing to fit your tax situation.

Actually, you can do even better, because now you should be thinking about 1982 and 1983 together. Since 1983 is to be a ZBA year, you can move some scheduled 1983 expenses back into 1982 and build even greater tax savings.

The system is simple. You just get into the habit of shifting deductible expenses forward from the preceding ZBA year and backward from the following ZBA year into the current "itemizing" year.

Unless you always have enough deductions to itemize every year, you get the most tax mileage out of your expenses by lumping as much as possible into the year you itemize and leaving the irreducible minimum for the alternate years in which you claim the ZBA.

Reminder: If you are eligible for an IRA for 1982, you have until April 15, 1983, to open an account and still deduct the amount of the deposit on your 1982 tax return. But the rules for Keogh (for the self-employed) are a little different. You can deposit funds on behalf of 1982 earnings until April 15--but the account must be in existence by Dec. 31, 1982. If you have an existing account from a prior year, you can wait to make 1982 deposits. If not, you must initiate the Keogh account by year-end with at least a token payment in order to claim a 1982 deduction.