Travelers are forever being told how to carry everything, from their clothes to their liquor. But what about their money? Traveler's checks? Letters of credit? Credit cards? Wads of foreign currency? A money belt? A trousers' belt that has a secret pocket on the inner side?

Anybody who watches television is familiar with the face of that law-and-order man with the snap-brim felt hat warning the world about pickpockets who prey on tourists. He is selling traveler's checks, and he says the advantage of checks is that you can get them replaced immediately, if not sooner, should your pocket be picked, your lock jimmied or your handbag stolen.

An artful crook could even steal your money belt, in which case you lose not only your money but also your pants.

Traveler's checks - or cheques as some prefer to spell them - come in a variety of flavors, including those issued by Thomas Cook, which started the whole idea; by American Express, which probably does the biggest business; by Barclays Bank; by Perera, the money-changers, and others. Not to be confused with any of these are Hilton-Cheques, which are prepaid coupons that quarantee the price of a hotel room regardless of the fluctuation of currency or the shift in room rates.

The idea for traveler's checks is traceable to Thomas Cook, who also organized the first travel agency. Cook called them "circular notes," announcing their issuance in the April 1874 number of the "Excursionist and Tourist Advertiser," the first travel magazine (if you don't count the words left by Herodotus).

"For many years we have been urged by great numbers of passengers availing themselves of our tickets and hotel coupons to allow them to deposit their surplus money with us and give them some medium by which they would be enabled to draw it from our agents in various parts of the world . . . "

With that 19th-century fanfare, Cook launched his program in New York, where his agents issued "circular notes" in denominations of 5 and 10 pounds. The first traveler's check looked like a bank note and was addressed, in fancy printing, "to the agents, bankers & Hotel Proprietors mentioned in our Letter of Indication." The inscription stated that the note would be "handed to you" by the client whose signature appeared below. The agent was requested to pay the bearer the noted amount at the current rate of exchange.

Cook now issues its checks in six different currencies: U.S. and Canadian dollars, Australian dollars, Hong Kong dollars, Japanese yen and Swiss francs. It used to issue checks in rupees for the benefit of traveling Indian princes.

Those who hold a Diners Club card, Amoco Torch or Sun Diners card can get Cook checks free of charge at any Cook office in the U.S. or Canada.

Some financial wizards point out, however, that when changing unused foreign currency or traveler's checks issued in foreign currency back to dollars at the end of a trip, a service charge will probably be levied.

Among the new wrinkles in the check business is Cook's Anytime Line, an 800 number (800-223-7373) that operates around the clock, seven days a week. Those who have lost checks, one way or another, are directed to the nearest office or agent of the company where an immediate refund of $250 is made, the balance usually refundable in 24 hours.

In the "What's New?" department there is American Express' latest offering of the Express Pac, a prepacked clutch of checks containing three, five or 10 checks, color coded. The application form only requires filling in the date. The customer can sign the checks while outside the presence of the seller. Tested for 18 months, beginning in the heavy-travel U.S. Bicentennial year, Express Pac drew only one complaint from 70,000 transactions. The new package system was put on the market this month.

The amount of traveler's checks bought but not yet cashed - and so-called float - reached a new high in 1977: $1.86 billion. This sizable amount of pocket money has included some lesser-known issuers of traveler's checks to offer them free. Deak-Perera International, dealers in foreign currencies, issues checks without charge in denominations of $20, $50 and $100 at its 50 offices around the world.

Free, too, at its offices in New York City, Boston and Chicago, are checks issued by Barclays Bank. These are also issued free to customers in savings and loan and in commercial banks. In California they are free to people who bank at Barclays.

Somewhat different, but a useful hedge against currency fluctuations, are Hilton Cheques. Purchasers of these checks are guaranteed a price of $45 for a single and $55 for a double in 36 Hilton hotels located in 22 countries. At six other hotels in London, Paris, Tokyo, Brussels, Rome and Vienna, there is a surcharge ranging from $10 to $20.

The hotel checks, available this year for the first time in the Pacific and the Orient, guarantee a room at the fixed rate through the end of March 1979. They are a hedge against currency fluctuations and rate increases necessitated by inflation. They also beat the cost of currency exchange. In the days of the anemic dollar having a guaranteed hotel rate in the pocket is as good as having an uncle in the hotel business.