Some great tips here - I'd like to add a few of my own:
1. If you decide to go to a grocery store be sure to pick up one of the cheap, styrofoam coolers. You can then fill this with inexpensive (or at least LESS expensive) soda/water you get from the store and ice from the resort and you have a do-it-yourself refrigerator.
2. Buy individual plastic bottles of water at the store, and get them as cold as possible the night before (freeze them if you can) - this way they will keep cold all day as you walk around the parks and they defrost.
3. Try to bring as much stuff from home as possible (rain ponchos, water spray bottles, sun tan lotion, film, batteries, etc.) - these items can really add up when purchased at WDW.
4. Choose your meals wisely - buffets are great ways to fillup and you can almost get "two meals for the price of one" if you eat enough
. Also, since lunch is always cheaper than dinner, eat your big meal at lunchtime and snack or go back to your room for dinner.
5. Between now and the time you leave give everyone Disney Dollars for presents (i.e. birthdays, christmas, etc) - this way they can't spend it now and they will have more for souveniers while at the world.
6. Visit one of the Character Connection stores outside of WDW (in nearby outlet malls) for your souveniers. You will always find great bargains, as long as you don't mind "out of date" merchandise - but what the heck - it still says WDW on it.
Hope these help
John