Ever try paying cash when renting a car? You can't. Hotel clerks now ask for your credit card as a prerequisite for checking in. Airlines will still take cash, but sometimes there's a long wait while they scrounge for the proper change.

In the travel business, plastic has become, literally, the means of transportation. That credit cards have become an American way of life is not surprising. However, in the last year, banks and other financial institutions, airlines, cruise companies and others in the travel business have begun to offer separate credit cards, vouchers or travel discounts.

As a result, a new generation of credit cards and financial services has emerged, geared specifically to travelers. The new cards and services expand your potential line of credit and also help you preserve the line of credit on cards you already have. They can speed up travel transactions and help you keep track of travel expenses separately from other expenses. Here is a sampling of many of the cards and services available:

*Airline credit cards. Most major airlines -- including American, United, TWA, Pan Am and others -- and many smaller carriers now market their own credit cards, with individual lines of credit.

TWA calls its card the "Getaway" card. Eastern offers a "Wings" card. A "Let Yourself Go" card buys you tickets on Continental Airlines flights.

These airline cards are now becoming very popular, although some have been around for a while. In fact, American's credit card has been available since 1948.

*Other credit cards. Airline credit cards are just the beginning. Royal Cruise Lines has introduced its own high-limit credit card. And even Sears has entered the travel credit card market.

Sears is introducing its new "Discover" card in a few cities in the South. It is described as a savings, spending and investment card. It is also a travel card. Discover cardholders can buy a washer-dryer and also charge travel on American Airlines. And the card will be accepted by Budget Rent-a-Car, Denny's restaurants and the Hospital Corporation of America.

The card will allow you to cash checks at Sears stores across the country, and First National Bank of Atlanta has agreed to let its 111 automated teller machines be accessed by the Discover card. (Sears expects the card to go nationwide by 1986.)

Recently, Royal Cruise Lines stunned the travel industry by announcing the company was issuing its own credit cards. It was a surprising move because no cruise line has ever done this before -- issuing a specialty credit card for purchasing expensive luxury cruises.

"We seriously thought about doing this for about nine months," says Duncan Beardsley, RCL's senior vice president of marketing. "So many of our passengers told us they wanted to use credit cards to charge their cruises that we created our own card."

Of course, anyone can now use a MasterCard or Visa to charge a cruise with Royal or other lines. "But that wasn't always a good deal," says Beardsley. "If you had a MasterCard or Visa and used it to pay for your cruise, you could easily burn up your line of credit. What this card offers cruise passengers is an entirely new line of credit."

And, with the average cost of most Royal Cruise itineraries -- which are worldwide -- that line of credit can be substantial, ranging from $8,000 to more than $20,000, with up to 36 months to pay.

Travel vouchers and related bonuses. Visa and MasterCard are now offering special travel voucher programs. Diners Club is offering frequent travel rewards. And American Express is using its card to market travel and luggage insurance.

Visa and MasterCard's vouchers, which come in the currency of the country where you plan to use them, allow you to prepay for hotels, rental cars and other travel services. For example, when a hotel asks for a deposit for a room reservation, you can get a voucher in the appropriate currency from your bank and send it instead of charging the deposit to a credit card or sending a check.

"Vouchers are typically purchased in advance and we guarantee the exchange rate," says Alan Schultheis, senior vice president of marketing and advertising for MasterCard. "If two months in advance your travel agent tells you your trip will cost $2,000, when the time for your trip approaches it may not cost 2,000 U.S. dollars anymore." (In other words, you'll be protected if the dollar goes down after you buy your vouchers, although you may not get the best exchange rate if the dollar goes up after you purchase them.)

"When you use your voucher you preserve your credit line for shopping while you travel," says David Brancoli, a spokesperson for Visa, U.S.A.

Visa travel vouchers will be available in November from travel agents in more than 50 countries. MasterCard vouchers are being tested later this month and will be available in October in l50 countries.

If you have a telephone, and your long-distance bills are substantial, AT&T assumes you must also like to travel. As part of its "Opportunity Calling" promotion, any month that you make $15 or more in AT&T long-distance calls, you receive an "Opportunity Credit" for every dollar of calling. You can earn up to 300 of these credits per month, and each credit is worth $1 in savings.

Currently, AT&T is offering some great travel deals. You can save up to $100 per person on selected Norwegian Caribbean cruises, get a free night at Stouffer hotels, save $100 on any Western Airlines round-trip ticket of $450 or more or save 40 percent on any full-fare adult coach ticket on Ozark airlines.

*Travel-related services. Citicorp Diners Club, which has consistently run a distant second to American Express in the number of cardholders, has made some elaborate changes to keep up with the trend in travel credit. Diners Club has now aggressively introduced a "New" Diners Club to its 5 million members worldwide. It claims to be a totally new "credit product" with seven new travel-related services.

"We found that business people [who are the card's primary users] are resigned to the inconveniences associated with travel because they do not see any alternative being offered, yet it was clear they wanted something more," says Citicorp Diners Club President James Emshoff. "Diners Club's new exclusive family of services was designed to bring a new level of convenience and productivity to business people both on the road and at home."

The program offers "Club Suites," luxury suites at hotels like the Mark Hopkins in San Francisco or the Ritz-Carlton in Boston for the price of a standard room. To eliminate waiting in line for a taxi you can use the "Club Chauffeur" program. For a few dollars more than a taxi, a chauffeur will greet you at or near the arrival gate, help you with your luggage and then drive you to your hotel. (It's a great service, but currently it is only available in l0 markets -- New York, Chicago, Los Angeles, San Francisco, Dallas, Houston, Denver, Boston, Atlanta and Washington, D.C.)

"Club Workout" allows you to pay a nominal guest fee and use all of the facilities at a select group of private-membership health clubs. It is available in the same l0 major cities.

"Club Business Offices" gives you the opportunity to rent up to 60 fully staffed offices in 46 U.S. cities. The offices are fully equipped and offer secretarial services as well as conference rooms.

Last but not least, Diners is trying to compete for travel dollars by offering something called "Club Rewards." This program allows you to turn dollars spent on travel into points -- $1 equals one point -- that are redeemable for free gifts and travel.

It all sounds good, until you try to add up points to get your "rewards." For example, if you accumulate 2,000 points (or $2,000 dollars spent using your Diners Club card) you can get a small box of Godiva chocolates. That's an expensive box of sweets.

If you want one of the top prizes, you'd better pray that a very wealthy corporation is reimbursing you for your travel purchases. An African safari for two can be had for only 925,000 points. And an eight-day trip to Monte Carlo for two, including air fare, hotel, two days on a private yacht and $5,000 in spending money, will require a mere 2,000,000 points.

*Extended credit on regular charge accounts. Many Visa and MasterCard cardholders are now being given substantial increases in their credit limits, meaning they can better handle large travel ticket purchases.

Diners Club offers a "Club Plus Account" of extended credit for card members in good standing. The account allows a member to extend payment on travel charges of more than $100. The accounts range from $2,000 to $50,000 in extended credit and have an annual interest rate of l9.8 percent.

The American Express card offers a similar program called "Sign and Travel." The service to extend payment on prepaid land, air and cruise travel arrangements is available to all cardholders. They must pay a $25 minimum and then payment is extended for up to 20 months along with the regular monthly finance charge.

The thing to remember about all these new travel credit cards is that having them is one thing, playing them right is another. There's a reason why so many cards are now being offered: Banks and credit issuers can make huge profits on annual card fees and interest rates that can push 22 percent for what is essentially an unsecured line of credit.

As with any form of credit, before you're tempted to take that flight or that long cruise, remember that getting there might be half the fun, but sooner or later you're going to have to pay for it.