4. Watch for price drops.

Just as with flights, cruise fares fluctuate as the date of the sailing approaches. If you find a lower price for your sailing for the same cabin category, most major lines, including Carnival, Royal Caribbean, and Norwegian, will honor the lower price even if you've already booked your cruise. All you have to do is fill out a price protection claim (sometimes called a best price guarantee form) or make a call to your booking agent. The difference in price is put into your account in the form of onboard credit, which you can use for things like wine with dinner, massages at the spa, and cocktails by the pool.

Some cruisers use this booking strategy to their advantage, booking way in advance and keeping a very diligent eye on sales — raking in the credit each time they find a lower price (which could be several times before the final payment is due).