A helpful hint would be to book your spa appointments early, specifically if you're interested in recieving service on formal night days. I found them to fill up quite quickly, unless I book the day of departure!
Sometimes you can sign up at the spa the first day and be notified throughout the cruise when there are deals. The spas typically have discounts on port days...
In general, I've found (not surprisingly) that you get longer, better spa treatments on land than on a ship, but I'm sure everyone's had different experiences, and obviously you expect to pay more for less when you're on a ship... Just something to consider going in. Just my two cents.
Booked on Carnival Freedom, Feb. 2012 (Cozumel, Costa Rica, Panama)
NCL Gem, Sept. 2011
NCL Jewel, Feb. 2011
NCL Dawn, Sept. 2010
NCL Spirit, Feb. 2010
NCL Gem, March 2009
RCCL Explorer of the Seas, Feb. 2008
Carnival Miracle, May 2007
Carnival Conquest, April 2006
Caribbean Princess, Feb. 2005