Those who claim you can't get something for nothing may never have attended a free financial seminar. Sponsored by such diverse organizations as Cartier, Mazza Gallerie and the Women's National Bank, these seminars feature lectures by financial experts on topics such as budgeting, investments and planning for taxes.

They are geared generally to the layperson -- from those with some financial savvy to others whose boldest attempt at high finance is balancing a checkbook. The sponsoring companies offer the seminars both as a public service and, of course, to draw customers to the company or lecturing experts. Some charge a nominal fee to cover costs and to gauge attendance.

At a recent seminar on "Personal Money Management" at Mazza Gallerie, Dean Witter Reynolds account executive Stuart H. Kershner offered these financial tips: :Investments

"If there are two people working, you don't want to leave too much money in the bank. But only certain investments make sense, depending on the couple's income.

"If a dual-income couple in a 50 percent tax bracket has $20,000 in a 9 percent T-Bill Plus certificate, which are very popular right now, they will make $1,800 a year in interest, which is 100 percent taxable.

"If they live in D.C. they'll end up paying $900 to the federal government and $198 to the District government. On $1,800 they'll keep $700 -- a 3 1/2 percent return. You tell me who wins."

Kershner suggests different types of investments for high- and low-income couples. He considers couples to be "high income" if their combined earnings are greater than $27,000, the average income for a Washington area family. Couples whose combined earnings are less than $27,000 might consider his recommended low-income investments.

If you are in a high tax bracket [30 percent and above], Kershner recommends investing in: 1. Municipal bonds 2. Tax deferred annuity 3. Real estate 4. Pension and retirement plans 5. Oil and gas drilling [for those in the 50 to 70 percent tax bracket]

For low-income couples who pay less than 30 percent of their combined income in taxes he suggests: 1. Money market funds 2. U.S. government bonds 3. Corporate bonds 4. High-yielding income stock [such as utility stock] 5. Certificates of deposit Separate Estates

While most couples own assets jointly, they might be able to leave their survivors more money if they divide ownership into two separate estates, says Kershner. Although each couple's financial situation varies, most couples with combined net assets in excess of $400,000 would leave more behind if they split their assets and willed them to the surviving spouse.

For example, take a couple with a jointly-owned estate valued at $750,000 and one spouse dies. Under the Tax Reform Act of 1976, the allowable marital deduction is 50 percent of the total estate or $250,000, whichever is more. So $375,000 would be subject to estate tax, which would equal $113,300. Subtract the $38,000 allowable estate tax credit and you get a total of $75,300 paid in estate taxes.

Had the assets been split and owned separately, the estate of the deceased spouse would equal $375,000. With a marital deduction of $250,000, just $125,000 would be subject to estate tax, equalling notes that dual-income couples often get so involved with their careers they tend to think money will take care of itself.

Figure assets and liabilities and decide on some short- and long-term financial goals. Set up a workable budget that allows you to meet both. Revise your goals annually. Lydon re-evaluates her financial goals in January for a monetary New Year's resolution. Upcoming Financial Seminars

"The Real Estate Game -- Should you Play to Beat Inflation?" by Stuart Kershner and James Sonnenfeld of Dean Witter Reynolds, Inc., tomorrow, Mazza Gallerie, free, call 966-5776 to reserve a place.

Brown Bag Seminars, sponsored by the Woman's National Bank, lunchtimes at the YWCA, 17th and K Streets, $1 each, call 466-4090 to be put on the mailing list for the fall series.

Financial seminars, sponsored by Ferris and Company, free, call 293-4500 to be put on the mailing list for the fall series.

Small business administration and Service Corp of Retired Executives (SCORE) Business and Finance Seminars, call 653-6979 to receive a calendar of upcoming seminars.