Boy are you at the right site! We are all about saving money!
Drinks in the parks, we bring water and juice in our backpacks, but I really don't know what the official policy is. A great site with money saving tips is
www.mousesavers.com
You will get more response if your question is more specific. But here are a few tips that come to mind right away,
When buying food at counter service eateries they often have only combo's on the menu but you can always order things individually for less. We often split meals to save money (and who wants to over eat and then walk around all day in the heat?)
If you are staying off site but have park hoppers you only need to pay one time a day to park, save your receipt. Also if you are going to eat at a resort you can park for 2 hours for free.
What are your plans, who is going (ages), how long will you be there, will you only go to WDW parks or are you going to US also, where do you plan to stay, did you book a package vacation or do you need tickets... The more we know the more we can help.