When it comes to taking a cruise, the early bird is not the only one who catches the worm.

Cruise passengers are slowly but surely realizing they don't have to pay full tariff for a cruise. You can save money by booking early, booking late or booking through a high-volume travel agency that gets a break on prices by virtue of the quantity of cruises it sells.You may also be eligible for a discount if you are a repeat passenger on the same cruise line.

Here are the details.

BOOKING EARLY: Most major cruise lines offer discounts if you book in advance. The percentage discount, which may range from 5 to 15 percent, and the advance notice, which might be three months, six months or a year, vary from one cruise line to the next. Early booking discounts are available through any agent that sells the cruise. By booking early you are assured the sailing date and cabin of your choice. The cruise line, however, is tying up your deposit for that length of time.

BOOKING LATE: What are referred to as cruise clearing houses, as well as some travel agencies, offer substantial savings if you're willing to book at what is considered by the cruise industry as the last minute - two to three weeks in advance on sailings with domestic itineraries and one to 1 1/2 months for foreign destinations.

"Cruise lines offer cabins to us at a discounted rate," says Duke Butler, owner of Spur of the Moment Cruises in Culver City, Calif. The company specializes in selling last-minute space. "You get the cruise for the same price whether you buy it directly from us or from a travel agency that buys it from us."

The idea, which has spawned numerous offspring, dawned on Butler when he was taking a cruise. "The ship was over half empty," he says.

Examples of what he has to offer include a seven-day Caribbean cruise for $599 per person, double occupancy, excluding air fare; a 17-day Panama Canal cruise for $1,599, per person, double occupancy, including air fare; and a seven-day cruise to Mexico starting at $499, per person, double occupancy, excluding air fare. Those rates, he says, represent a savings of approximately 40 percent.

Cruise lines hope clearing houses will sell cabins that would otherwise sail empty.

Clearing houses publish a list of discounts that are available in the near future. For information you must contact the clearing house directly or work through a travel agency that can purchase tickets from the clearing house.

In the cruise industry discounting is usually discussed behind closed doors. "Most of the cruise lines will not talk to anybody about discounting," says Butler. "They don't want to talk about us, but they need us to top off their ships."

Information about clearing houses and their toll-free phone numbers may consequently be difficult to come by, especially if you are going on a cruise for the first time. Many discounters, however, advertise in magazines that specialize in cruises and travel sections of newspapers.

A word of caution: Booking a cruise at the last minute is not always as simple as it seems. Things to be aware of include the possibility that the last-minute discount may not materialize, you may not get the sailing date you wanted or the cabin category of your choice. And many last-minute bookings do not include air fare.

"Most of us know that the closer to your date of departure you book your air, the more you have to pay," says Debbie R. Adams, president of the National Association of Cruise Only Travel Agencies (NACOA) and owner of The Ship Shop, a cruise-only agency in Kansas City, Mo. "After you purchase your own air ticket and pay for your transfers from the airport to the ship, you may end up in a non-saving situation.

"If you purchase the cruise line's entire package, including air fare, it knows where you are and what flight you're on. If there's a delay it will wait for you. If you've made you're own arrangements and you're delayed, the cruise line doesn't know where you are. The ship might sail without you," she says.

Be leery of an organization that asks you to pay money up front before you can receive information about discounted cruises. "We carry advertisements from many, many cruise-only agencies and clearing houses," says the editor of a cruise magazine based in the Midwest. "A few ask you to pay money up front and we've had complaints about some of them. Those seem to be a little trickier."

BOOKING THROUGH A HIGH-VOLUME AGENCY: They pass on savings to passengers that the cruise lines allocate to them because of the quantity of business they do.

"In preferred supplier relationships a cruise line will give a travel agency a special rate or considerations," says Janet Lynch, editor in chief of "Cruise Views," a new magazine geared especially toward travel agents. "Many cruise selling agents who do a lot of volume can match the discounts offered by clearing houses."

Cruise Time, in San Francisco, Calif., is one cruise-only agency. "We do a high volume and we only sell cruises," says general manager Paula Basker. "Cruise lines allow us to discount many of their sailings."

According to Basker the discounts average 20 percent. "Once in a while we can give you discounts up to 40 percent."

The company also sells some Sea Saver (last-minute) discounts, but as is the case with clearing houses, air fare is often not included.

Many agencies that do a high volume in cruises and are therefore eligible for supplier discounts are members of the National Association of Cruise Only Agencies. For a list of NACOA travel agencies, send a self-addressed, stamped (45 cents) envelope to NACOA, P.O. Box 7209, Freeport, NY 11520. Specify up to three states in which you would like to know the names and addresses of NACOA agencies.

But NACOA agencies aren't the only ones who can sell cruises at a discount. "Because NACOA agencies specialize in cruises they should be able to offer you a good price," says Debbie Adams, NACOA's current president. "However, there are travel agencies that may not belong to NACOA that can offer you good prices, too."

REPEATERS CLUBS: Many lines offer discounts to passengers who have sailed with them before. Repeat passengers are also invited to special parties hosted by the captain. A repeater's discount can rarely be combined with another discount and is generally not as generous as the savings you can get from a volume agency or clearing house.

Some people argue that price is not the bottom line in choosing a cruise. You should also consider the destination (is it some place you really want to go?), and cruise line (some cater to the young and energetic; others to older, more sophisticated people), and the sailing date.

We suggest you then make a few phone calls to compare prices. "When you find exactly what you want - the vacation you want on the ship you want at the time you want - at what you consider a fair price, buy it," says Adams. "Cruises are not an over-priced product even without the discount. It's not a sin to pay full price if you're getting good value for your money."

On the other hand, paying full price for a cruise is like paying sticker price for a car. Nobody needs to do it.