Thanks to a mixture of holiday-season generosity and enlightened self-interest, many people give some extra thought at this time of year to charitable contributions.

Donations bring the psychic reward of helping out your favorite social service agency, religious organization or college.And they also offer the financial reward of reducing your income tax bill, since most contributions remain fully deductible under tax reform.

If you have any doubt about whether a specific institution or program entitles you to a deduction, you can check your local library for Internal Revenue Service Publication 78, "Cumulative List of Organizations."

Whenever you contemplate charitable giving, experts in personal finance say, it makes sense to know the tax rules so that both donor and donee get the greatest possible benefit.

For one thing, you must itemize your deductions in order to take advantage of the tax break. A special arrangement that was set up for non-itemizers a few years ago no longer exists.

So if you are in a situation where you itemize in some years and take the standard deduction in others, it makes sense to consider concentrating your contributions in years when you plan to itemize.

The simplest form of contribution is cash, which can be documented either by a receipt or canceled check. But that is by no means the only way to do it.

"If cash is not readily available, it is possible to charge donations on a credit card and take the deduction in 1988, while deferring payment until 1989," says the Arthur Young accounting firm.

Many organizations accept non-cash contributions such as clothing, books or canned goods, which you can deduct at their fair market value.

"Specific records must be kept to support a deduction for donated property," cautions Deloitte Haskins & Sells, another large accounting firm.

If you claim non-cash contributions of more than $500, the rules require that you file a special form with your tax return to document it. Above $5,000, you also must provide a formal appraisal, unless the item involved is a publicly traded stock whose value at the time of donation is a matter of public record.

Stocks and other capital investments that have appreciated in value since the time of their purchase qualify for special treatment when they are given as charitable gifts.

Donors of these securities can take a deduction for the full current market value, and avoid a capital gains tax liability at the same time. Higher-income people giving substantial amounts of appreciated property are advised to check beforehand to see whether this will expose them to the alternative minimum tax.

There is nothing to prevent you from contributing a security on which you have a loss. But the experts say it is smarter to sell such a security and then give the cash proceeds to the charity of your choice.

That way, you can write the loss off on your tax return, within limits set by the tax code, as well as take a deduction for the security's market value.

What if you prefer to contribute your time, rather than money, doing volunteer work? The rules don't permit you to assign a financial value to your time and deduct it.

However, say accountants KPMG Peat Marwick, "you can take a deduction for your out-of-pocket expenses incurred in donating your time, and for the charitable use of your automobile."

The standard rate for car use is 12 cents a mile, plus any parking or toll fees you pay. If you are willing to keep good records of all your auto expenses, you may be able to take a bigger deduction.