I use a combination of what others have mentioned, but here is one I didn't see listed.
Pick a denomination of paper money and every time you have one, put it away. I chose $5 bills. Every time I get a $5, it goes into our vacation fund. You would be surprised how quickly it can add up.
Coins in the jar - we do this as well, although we don't seem to have as much change as PP mentioned!
Tax Refunds, bonuses from work or dividend checks - these are cashed and stashed as well.
Consignment Stores/Craig's List - anything we sell at these places (or have a garage sale) - that money also goes into the vacation fund.
Disney Visa - we use this for EVERYTHING and pay it off every month. Before we cruise (or go to WDW) we get the rewards card and that is used for spending money on the ship.
When you are ON your cruise - BOOK ANOTHER CRUISE ON BOARD!! You will get a 10% discount and OBC (on board credit). You can choose a "dummy date" if you don't know when you want to cruise again. Then you can move that reservation to a future time and still get that discount. In addition, use a
travel agent that also offers OBC. I use
Dreams Unlimited as our TA (you have to name them when you make the reservation on board) and got $200 in OBC from them in addition to the $200 in OBC I got from Disney for booking onboard (7 night cruise).
Good luck with the saving!