Definitely check out the Entertainment book for coupons. Just be careful - sometimes you can find cheaper rates using another source.

You can search this forum for other FTers favorite booking sites. I can suggest cruiseleaders.net. I tend to find that they consistently have the lowest prices.

Lastly - if you have previously sailed with a particular line...call them directly. I have found (on several occasions) special pricing that I couldn't find anywhere online.

Good luck!

-Chris