Chances are you've been hearing a lot about new federal tax reform proposals. Whether they wind up in the congressional shredder or not, you still, no doubt, have the hefty 1984 tax packet to fill out. Don't overlook the sales tax deduction, which can save you money.

Sales tax is a daily, "invisible" tax. You shell out 4, 5, 6 or 8 percent in sales tax on almost everything you pay for, and have nothing to show for it. Or do you? If you save receipts -- and if you don't, start now -- you may be able to increase substantially the size of your sales tax deduction on Schedule A of Form 1040.

Don't neglect any possible source that documents sales tax deductions. Nail down the big ones first: Schedule A has a special line for deducting sales taxes on big purchases such as cars, trucks, boats, airplanes, motorcycles. Moreover, as IRS spokesman Ernest Acosta points out: You can deduct sales tax paid on prefab or mobile homes and on materials purchased to build or improve a home (if taxed at the same rate as the general sales tax). For future purchases, be sure to ask that your receipts or contracts show the sales tax separately.

If you borrowed to finance a large purchase, you may deduct all the sales tax in the first year (but only the finance charges you actually paid that year).

What about ordinary, everyday purchases? If your disposable income goes toward kids' socks instead of stocks, you're probably shortchanging yourself by using the Optional State Sales Tax Tables provided in your federal tax packet. Just as the name indicates, they're optional -- not mandatory. "If you kept records that show you paid more sales tax than the table for your state indicates," concedes the IRS, "you may claim the higher amount."

The IRS and other tax authorities believe, however, that most people don't bother to save their receipts; therefore, each year's estimated state sales tax tables are based on three simple factors: income, family size and local taxing rates.

For example, a single individual in the District of Columbia earning a 1984 total available income of $28-30,000 could deduct $232, corresponding to taxable purchases of $3,867. This is only about 13 percent of income. In spite of inflation, this figure has not been increased since 1981.

A D.C. family of four reporting a 1984 total available income of $28-30,000 could deduct, according to the IRS table, $317 in sales taxes. This corresponds to a total expenditure on taxed goods -- clothing, furniture, household goods, appliances -- of only $5,283, less than 18 percent of adjusted gross income. (Financial adviser Sylvia Porter recommends limiting consumer debt to 20 percent of net income.)

A D.C. family of five with the same $28-30,000 income could deduct, using the IRS table, only $13 more than the family of four (for a total of $330).

In Maryland, a family of four reporting $28-30,000 would be allowed a 1984 deduction of $270, corresponding to taxable purchases of only $5,400. And in Virginia, where food purchases are taxed, the same family with the same income could deduct only $298 -- although a family with growing teen-agers might reasonably expect to spend $100 a week for groceries, paying a yearly sales tax of $208 on food alone.

If you -- as the IRS assumes -- have not been hanging onto your sales slips, how can you find out whether your sales tax deductions will exceed what the optional sales tax tables permit you to deduct?

"A good starting point is to go through the year's worth of canceled checks," says Kenneth J. Shapiro, a CPA/attorney and tax director of Kaufman, Rosenbloom & Shapiro, Bethesda. "Pick out all the items that might be taxable purchases. Then review all your charge accounts -- the vast majority of charge card purchases are subject to sales tax."

To make a quick and easy estimate of your sales tax deduction, add up those checks and charge account payments, dividing the total dollar amount by 1.04 if you're a Virginia resident, 1.05 for Maryland residents, 1.06 for District residents. Then subtract the result from your original total. This figure will give you a rough estimate of the sales tax you paid via checks and charge accounts..

Remember, this is only an estimate and may not be adequate in the event of an audit: You might have lumped in untaxed items, plus some taxed at different rates. But it will give you an idea of the savings you can expect by keeping good sales tax records.

If you live in Virginia, it will be especially important to save your grocery receipts, since your food is taxed. But other area residents also pay sales tax on nonfood grocery items such as cleaning supplies and pet needs.

"Make a habit of putting receipts in a special place," advises the IRS' Acosta. "Keep types of receipts together and circle the sales tax figures in red" to make totaling easy later.

Another suggestion comes from Maryland Income Tax Division spokesperson Marvin Bond: "Organize. Spend $2 for an accordion folder and throw the grocery slips in one pocket, and so on. Then you have it all in one place to figure the deduction, and if you're audited you have the slips to prove it."

Once your federal return is complete, you can reap even more savings on your Maryland, Virginia or District return: All allow deduction of sales tax. "The rule of thumb," says Bond, "is that any item deductible from the federal return is also deductible from the state return."

For more information on sales taxes and tax policy:

IRS Taxpayer Service, 488-3100.

D.C. Sales Tax Information, 727-6104.

Maryland Income Tax Division, 949-0280.

Virginia Department of Taxation, 534-5791.

Finally, what to do with your savings from sales tax deductions? You might want to spend your bonanza on professional tax preparation next time around.

The tax preparer's fee, don't forget, is also deductible.