My biggest money saving tactic is doing ports on our own. (Unless it is dangerous not to do so) We always see more for cheaper than ship sponsored excursions.
I do laundry on the ship if they have self-service and bring Purex sheets which have the soap and fabric softener in a sheet together.
Also, finding a good bartender that you are loyal to can save you money in drinks because the drinks may be stronger, leaving you to purchase fewer. I have also heard of people getting slipped some for free.
I tend to not frequent the ship's spas because of the expense and the high pressure sales pitch.
We sometimes also book a lower category cabin with the knowledge that the money we saved, we'll spend on specialty restaurants etc.
Soda cards usually save us money because we drink so much soda. (We could save more by sclepping it on, but never do for convenience)