Those little pieces of plastic you get from your bank or from companies like Diners Club and American Express can save you money on your next foreign vacation-if you play your cards right.

The secret is knowing when to use them, says James Stetler, executive vice president of marketing for Diners Club.

You're ahead by using a charge card right off the bat, says Stetler, because you don't have to pay the bill right away and your money can be in the bank earning interest.The thing to do if you're planning a foreign vacation is to take several cards, but do a little checking on your charges before you leave.

The most important specifcs to find out are:

Which cards charge for currency conversion, and how much (some do and some don't).

What are the billing dates on the cards.

Do any of the cards offer free benefits to the traveler.

"Many vacationers don't realize that when they convert dollars into foreign currency even at banks, they lose 1 to 2 percent every time," Stetler says. "If you're on a tour of several countries, you can easily be out 8 to 11 percent converting back and forth at banks, and up to 30 percent if you exchange your dollars at hotels and stores."

"With charge cards," he says, "only the amount charged is converted to the foreign currency and you won't be left with a lot of francs, marks, etc." In addition, he says, some card companies don't charge commission on conversions, so you save even more; and you can save more yet, because most card companies buy foreign currency at a bulk discount rate onto their card holders.

As far as travelers checks go, they're better than cash because they can be replaced if they're lost or stolen. But Stetler feels they should only be used as emergency cash.

"You still get clipped on the conversion," he says, "though not usually as much as you do when you exchange dollars."

The reason for taking several cards and for checking their billing dates is so you don't get hit all at once for the entire charges you run up on your vacation. "Some companies bill you on the first of the month, others on the 15th," Stetler explains. "It makes sense to mix your card charges so you get the longest possible run for your money before you're billed."

Also, he says, be sure to check to see if you'll receive any special benefits by using a certain card. Some offer free accident insurance if you charge your airline, train or other transportation tickets to that card. Others offer free or discounted travelers checks to card holders.

There are two major types of credit cards-bank cards, and travel and entertainment (T&E) cards. The largest T&E cards are American Express and Diners Club. Both types have their benefits and drawbacks.

The big benefit in bank cards is that they cost nothing to get. The drawback, according to Stetler, is that if you don't pay the the entire balance when you receive your monthly statement, you are hit with an interest charge on the balance of as much as 18 percent annually in some states for the first $500 owed and then 12 percent for the remainder of the borrowed money.

"That's the way bank card companies make most of their profit," he says, noting that both bank and T&E cards also get a commission from the establishments they service, from 3 to 4 percent for T&Es, and less for bank cards.

Bank cards also have a credit limit of about $1,200. "Once a card holder reaches that sum, his spending power is limited. Therefore it's a good idea to take a couple of bank cards along on your trip if they are the only type of card you intend to use."

The drawbacks of T&E cards are that you have to pay to get them- $25 to $30 a year-and you can't get them unless you mett certain minimum annual income requirements-$12,000 to $12,500 depending on the card.

However, there are also numerous benefits.Diners and American Express each offer some exclusive services to their members. Both also offer some benefits; these include free travel accident insurance when you charge your tickets; confirmed hotel reservations; no credit limit on charges; and a credit line of from $2,000 to about $15,000 at an annual rate of 12 percent interest.