ANYONE PLANNING an international trip this summer should be sure to carry a supply of foreign-currency travelers checks along with their U.S.-dollar checks. That's the cheapest and most convenient way to take money abroad, say travel advisers.

Banks and other foreign exchange specialists now sell checks in most major West European currencies as well as Japanese yen and Hong Kong and Australian dollars. In addition to protection in case of loss, the advantage is twofold:

* No exchange fee must be paid to cash foreign-currency checks. In some foreign destinations, it can cost $2 to $3 per U.S. check to convert them to the local currency, a substantial bite.

* Local-currency travelers checks are more readily accepted by shops and restaurants, particularly during times of rapidly fluctuating exchange rates--as one Washington secretary discovered.

On her first trip abroad, to Paris, she took French franc checks as advised. After lunch on the day she arrived, she pulled out her checkbook to pay. "Non," insisted the waiter. "Non, non." The bewildered secretary headed for the cashier. "Non," she was told again, quite sharply this time.

Finally, calling up nearly forgotten high-school French, she struggled to say she carried checks in francs. At that, the staff all but swept her up in their arms in apologies for their misunderstanding. Of course, they said, her check was good. They simply had figured she was carrying U.S.-dollar checks like so many other American tourists.

The best bet is to figure up in advance hotel and meal costs for a particular country, and then buy checks for that amount in the country's money, advises Karyn Gardner, a Deak-Perera foreign currency specialist. For shopping and excursions, take U.S. dollar checks--but spend them last. In this way, the foreign checks are used up abroad, and the traveler returns home only with dollars, which, of course, don't have to be reconverted.

When cashing a U.S.-dollar check in a foreign country, remember that banks and approved currency exchange offices usually give a better exchange rate than hotels or restaurants. Shop around for travelers checks. Some firms do not charge an issuance fee, while others may charge 1 percent. The savings could pay for all those postcards home.

Before departure, also buy a small amount of currency for each country on the itinerary. As seasoned travelers know, this loose change comes in handy for taxi and bus fares from the airport, hotel tips and other arrival incidentals, especially if the exchange windows are closed when the plane arrives.

Big spenders who are leery of toting a huge wad of checks can buy U.S.-dollar American Express checks in $1,000 denominations to tuck into a money belt. When it comes time to cash them, American Express says it will break them down into smaller-denomination checks at no extra charge.

American-Express card holders needing money abroad can write personal checks for $1,000 to $2,000 in travelers checks (depending on the card) at the firm's offices in major cities. This solved the money problem for one around-the-world trekker, who was visiting 35 countries in one year and didn't want to carry a year's supply of money.

First, to protect against theft, he punched a whole in his plastic charge card and hung it by chain around his neck. Each time he ran low on funds, he would present the card at an American Express office and write a personal check for several hundred dollars. At the same time, he would notify his Washington bank by mail to transfer the amount from his savings account to his checking account. At the most, he was carrying only a few hundred dollars in travelers checks at any one time. Meanwhile, his travel money was still earning interest at home. The system worked flawlessly.

Travelers planning to stay in a foreign city for a lengthy period--for example, six months in Venice--have the option of buying a foreign bank draft from a Washington bank with international dealings. The draft, in any amount needed for living expenses, can either be wired to a bank abroad, or the traveler can take it along. At Riggs, the fee is $10 to $25 for non-depositors, plus a cable fee of several dollars. Some banks may offer the service free to good customers.

If an emergency arises while away from home, family or friends can wire money through cable services offered by international travel agencies such as Thomas Cook and American Express or by cabling or mailing a foreign bank draft.

Todd Stanton, branch accountant for Thomas Cook, says frequent clients are the worried parents of young travelers stranded abroad who have phoned Mom and Dad for extra funds to get back home. "It happens all the time."