You must claim a valid income-tax deduction before you can get it

By now, most Americans have those flimsy pieces of paper known as W-2s. These forms tell how much money one earned last year and how much was sent to federal and state governments, and to social-security accounts.

Now it's time to line up the W-2s and get ready for the annual exercise of filling in the tax forms. Whether you do it yourself or have a professional preparer or tax accountant do it, you should bring something else to the session: a list of deductions.

If you are eligible to itemize deductions, tax experts say, you should take every deduction you honestly feel you have coming. This does not mean you can simply decide something is deductible because your personal sense of justice says it should be deductible. There must be a legal and verifiable reason for the deduction.

If there is a reason, these experts add, people should not be afraid to take deductions because they are afraid of an audit. If a taxpayer is audited, he can often persuade an auditor of the legitimacy of a deduction, even if the auditor disapproved of it at first.

Keeping this in mind, however, remember that you have to claim the decuction first. Or, as one tax expert says, ''If you don't claim it, you can't deduct it.''

In addition to the usual reasons for claiming whatever legitimate deductions you can, there is a special reason for doing so this year. With lower income-tax rates in effect, your 1981 tax burden will be heavier than 1982. So you want to find as many ways as possible to lighten 1981's burden through deductions and credits.

There are a variety of ways to find out what deductions you have coming. The instructions that came with your 1040 tax forms from the Internal Revenue Service list many of them. Even though they are written in fine print and seem to be in a language that only faintly resembles English, they contain enough deductions to make reading them worth the effort.

Keep in mind, however, that the goal of the IRS is to collect as much money for the government as it can. So it is not going to list and explain every possible deduction. In fact, no publication can do that. Many deductions are not listed anywhere. They become part of the law because someone bothered to claim them and a tax court agreed with the taxpayer.

So if the IRS instructions do not seem helpful enough, the bookstores and newsstands these days are loaded with publications that list many deductions. A few of the books even explain the tax court background behind some of the deductions.

It doesn't take many newly discovered deductions to pay for the price of the book. In fact, Uncle Sam even does his bit to help defray the cost: The cover price itself is deductible. This is also true of any publication you buy or subscribe to during the year because it helps you with your tax questions or investments.

Even people who have scouted out every possible deduction sometimes forget to do an important tax-saving exercise: income averaging. Did you have a substantial increase in your income last year? This might have come from a promotion that raised your pay significantly, a second job you took on, a spouse who started working, capital gains from the sales of stock, free-lancing, gifts, or inheritances. The averaging will result in part of the extra income being taxed at a lower rate.

If this extra money made your ''averageable'' income more than $3,000 greater than 120 percent of your average income for the previous four years (1977-1980), you are eligible for income averaging.

While this might sound confusing, the IRS has a special form to help. If you think you might be eligible for income averaging, ask your IRS office or tax preparer for ''Schedule G.'' The form not only tells the IRS you are claiming this privilege, it also provides a step-by-step process for figuring out whether you qualify. It even stops you in mid-form if you are not eligible.

Should this exercise prove fruitless this year, don't throw your Schedule G away. You might be eligible next year and it will be easier to find out with some of the arithmetic already done.