You've found a killer cruise deal -- but you're worried that once onboard, you'll bust your budget.

It doesn't have to happen. A little advance planning -- and a little discipline -- can save the day. Here are our tips for keeping your budget in check:

• Hair, nail and toiletries: Get your hair cut and styled before you embark; a salon visit costs more onboard than ashore. And don't forget to bring your own toiletries (my wife once left her hair spray home and ship's salon charged her $15 for a can).

• Spa and fitness: Spa treatments are expensive. If you're on a tight budget, do without or book specials offered on boarding day and when ships are in port. Some ships charge for fitness classes or equipment use; be sure to ask before you sign up.

• Snacks and sundries: Bring them aboard. A little bag of chips will cost two-three times as much on board as on shore.

•  Drinks: Alcohol isn't cheap, though if you compare drinks on board with cocktails on South Beach you may be pleasantly surprised. Some ships include all nonalcoholic beverages with meals, some allow only tea or lemonade, so you'll want to check in advance. And if you tend to stick with sodas throughout your cruise, ask when you first board about buying a soda pass; this flat-fee card can be much cheaper than buying a la carte.

• Photos: Ship photographers are everywhere, snapping photos of you getting on the ship, at the ship's rail, at dinner, on shore excursions. Those pro snaps are posted in a gallery, where you can buy prints for around $9.95 each. But you'll pay nothing at all if you ask fellow passengers to snap photos of you with your own camera.

Ship photographers also set up studio shots on dressy nights that can be yours for $19.95-up -- not a bad price for a formal portrait you can frame at home.

• Gambling: How much you choose to gamble in casinos or at bingo is up to you, but bear in mind that the odds are always in the ship's favor, not yours.

• Alternative restaurants: They usually command an extra charge per person, so if you're on a tight budget, pass them by. On the other hand, the dining experience in these restaurants usually is quite a treat and worth at least a one-time expense.

• Internet: Ships charge high prices for computer use, but the service (by satellite) is often slow and frustrating. Usually you'll find Internet cafes near the piers at ports of call. You'll pay much less for usage than on board and connections are better.

• Telephone: Ship-to-shore calls are expensive, as much as $18.95 a minute. You'll fare better using your own cellphone. Note that you may have to arrange with your provider to have access to service outside the United States (I pay $5.99 a month) and you'll pay roaming charges (for calls from Europe, my cost was $1.25 to $2.49 a minute).

You can also place calls from a public phone at a foreign port of call, using your provider's special number to access a U.S. operator; then you are charged at U.S. long-distance rates, not those of the foreign country.

• Kid activities: Though many programs are included in your cruise fare, some involve an extra fee. Check before you sign up.

• Excursions: Some excursions, such as helicopter sightseeing, will set you back hundreds of dollars. Being on a budget doesn't mean you're ship-bound. Nearly all major cruise lines sell ship-sponsored excursions that offer an overview of a port of call at reasonable cost.

You can explore on your own more cheaply -- especially in ports where you just want to walk around. If you snag a taxi or book a tour locally, be warned that the responsibility is yours. The ship won't wait for you if you're late getting back.

• Don't get taken: Scam artists frequent the piers and touristy areas at ports of call. Be wary of people on the street or in stores offering deals too good to be true, because that's usually what they are.

• Shopping ashore: Before you leave home, decide what expensive items -- like jewelry or crystal -- you might want, and check the price online or in a local store. This will give you an idea whether the prices you see abroad are a good deal.

Be aware that ship-recommended stores usually are required to have a return policy, but may have somewhat higher prices. Other stores at ports of call may have lower prices but no guarantee of quality or return.

•  Shopping on board: Ship stores generally will meet any price ashore if you can prove it, but their stock is often limited. Still, you can sometimes get good bargains on board -- especially on ''sale'' days near the end of the cruise.

• Special services: Some ships charge for exclusive deck spaces, some for ice cream not taken in the dining room, some for onboard tours, some for use of fitness machines. Avoid art auctions.

• Tipping: Don't skimp. Ship service personnel work long hours to make your trip pleasant. Follow the ship-recommended guidelines for tipping -- about $10 per person per day -- or more if you feel you've received extra or superior service. Do not tip bar waiters; their tips are included in the price of drinks.