You pick up the envelope and immediately recognize what's inside.

"Oh no," you sigh, "not another one."

Hardly a charitable response to a plea for a donation, but who can blame you? Daily, it seems, somebody wants you to give to their cause.

One person complained of receiving 400 appeals last year, at least 15 from the same group.

"Dear Contributor," writes one recent appeal, "Your American Lung Association Affiliate is working every day of the year to help protect your lungs!"

"Should I contribute?" you ask yourself. It takes only one look at the murky air outside to know your lungs need all the protection they can get.

Or do you chuck it out, perhaps feeling just a little bit guilty that you haven't done your share for humanity?

Fall, say fund-raising experts, is a big season for charity solicitation (as is spring). Expect a fresh batch of appeals soon -- if not in the mail, then by phone, at the office or on the street.

Charity is big business in the United States. The statistics are awesome.

Americans contributed more than $43 billion in 1979, surprisingly almost 90 percent of it from individuals. Only 10 percent came from corporations and foundations.

About half went to religion, with the rest to education, health, social welfare, arts and other causes.

"It's always a big shocker to find our $5 and $10 add up to so much," says Helen O'Rourke, who heads the Philanthropic Advisory Service of the Council of Better Business Bureaus, which monitors fund-raising organizations throughout the country.

"Americans are generous people; they always have been," she says. Our average for giving "is just under $200 a day for every man, woman and child in the country. The nearest country is Canada at $35 per capita."

O'Rourke's office, working with charities, has established a code of fund-raising standards. Among them: disclosure of "all reasonable information," truthful advertising material, and the avoidance of harassment and intimidation.

If you have a complaint, inform her office or the 148 Better Business Bureaus nationwide. The bulk of the complaints, says PAS director Nancy J. DeMarco, are about multiple mailings, which donors consider "a waste." This occurs when a charity acquires several mailing lists with your name.

The office publishes a quarterly update, "Give But Give Wisely," on 400 most-inquired-about charities, listing those which do, or do not, meet these standards. It is available to the public.

O'Rourke's office has files on 10,000 fund-raising groups. Obviously, with that many, you're not going to be able to give to them all (though it's possible that one awful day they could all show up -- hands extended -- at your front door).

Many of us give haphazardly through the year, to a dozen or more groups, ranging from odd change dropped into the Salvation Army. Christmas bucket to substantial sums in the United Way office drive.

Is there a better way?

To find out, "Changing Times" magazine recently made a study of how the professional givers -- the foundations and corporate contribution offices -- make their bequests. The conclusion: Do as the experts do. Set aside a certain amount to give. Then apportion it out to the causes that interest you the most. That could be just one, or three or four.

With big money, there's potential for fraud. O'Rourke's office receives 150,000 inquiries a year, most of them asking: "Is this charity legitimate?"

But, she estimates, fraudulent charity solicitations probably amount to "less than 1 percent" of the total.

Well-publicized charity scandals in the last few years, says O'Rourke, have brought about "a big change in the attitude" of donors. "They're asking questions" about where they are putting their money as part of "our consumer age."

One of the biggest scandals involved the Pallottine Fathers, a Baltimore missionary order. They collected millions of dollars which donors thought were going to the poor. Instead most went into costs and investments.

The Pallottine disclosure, says O'Rourke, spurred many more religious organizations into meeting the Philanthropic Advisory Service's requirement for disclosure. "I think that's what shook them up."

Only a few years ago, says the service's monthly newsletter "Insight," these groups, when questioned about finances, often responded: "We are accountable to God for our use of funds."

A number of religion-oriented groups still refuse to provide information on their programs and finances.

Charities fail to meet the Philanthropic Advisory Service's standards if too much of their money goes into fund-raising, salaries and other administrative costs. The requirement is that at least 50 percent of the collected money must go to the group's stated charitable cause. However, says DeMarco, "we start asking questions" if that figure is not higher for established charities.

Newer charities must spend more to build a mailing list, points out O'Rourke, and that's "the most-expensive kind of fund-raising." And there are "unpopular diseases" that are harder to collect for, among them "epilepsy, alcoholism, V.D."

The nation's troubled economy, she adds, is also making it hard for charities as financially-strapped donors hold on more tightly to their dollars. c

O'Rourke, a one-time fund-raiser herself, and DeMarco cite a number of tactics donors should be aware of. One is the letter that makes you feel guilty.

"That's the way the letter is written," says O'Rourke. "It's usually geared to women, to the people who stay at home."

"Because it's so hard to raise money," adds DeMarco, charities "want the emotional appeal." But some recipients think they may overdo it. DeMarco has received complaints about "distasteful" solicitations involving pictures of "starving children and bloated bodies."

Some appeals may be disguised as a bill or urgent invoice. Others urge you to contribute before it is "too late."

In their bid for your bucks, charities have come up with innovative campaigns, with which PAS has no quarrel. But the appeal , says O'Rourke, can turn into a "gimmick" if it misleads or misrepresents: "Sweepstakes prizes that are not awarded, matching gift appeals based on a $10 challenge, survey responses that are not tabulated."

As for unordered merchandise some charities send with their bid for money, O'Rourke says, "If people didn't give, they wouldn't be in business."

Such mailings are especially expensive. PAS standards require the charity to disclose "that recipients are under no obligation to pay for or return any items received."

A telephone solicitor asking you to help a charitable cause by subscribing to a magazine or buying other items may be a "volunteer for a worthwhile cause," O'Rourke points out in her newsletter. Or it could be "a professional solicitor getting a fat slice of the take."

In some cases, "The sales pitch of a strictly profit-making firm implies a charitable purpose by mentioning, for example, the handicapped persons employed by the firm."

In the wake of a disaster or tale of hardship here or abroad, says DeMarco, "We see a real surge of appeals geared to that." Some are verifiable as legitimate. Others "won't tell anything to anybody."

When in doubt, advises O'Rourke, give to an established group such as the Red Cross or Care.

Be wary when you're donating to someone on the street carrying a canister, she says. "There's very little accountability."

Coin cards sitting at store check-out counters also may not be the best place for your money, O'Rourke found by first-hand experience. "They were broken into. The stores don't want to take the responsibility for them."

"Be very careful" if it's somebody you don't know, even children, collecting door-to-door. If a solicitor can't answer your questions, "Give to a charity that keeps volunteers informed."

O'Rourke warns of "Candy Caper Kids," who are trucked into a neighborhood to sell candy and candles for perhaps unworthy causes. "It's only a dollar here and a dollar there, but that could be going to a very worthwhile charity."

Similarly, handicapped peddlers (or those pretending to be) "may travel in groups under the direction of a boss who provides transportation, and who generally receives 50 percent or more of the take."

If you're doing volunteer work, she says, "Be as careful as if you were giving money. You're giving your time, which is money."

O'Rourke's organization never advises on who to give to, but she will name her favorite charity -- "the Salvation Army. It comes from years of seeing people go there for help. They help you first and ask questions later. sOthers make you fill out a form first."

Her most important piece of advice to donors: "If I could put a banner across the United States, that's what it would read":

"Always give by check, never by cash."