Make sure you pack what you are likely to need. Sunscreen, feminine products, extra socks, bandaids, tylenol, camera batteries, etc. These are the things that when you realize that you left them at home cost an arm and a leg in the gift shop.
Buy a bottle of liquor and mixer in the resort gift store rather than getting pool drinks. Still cheaper, don't drink alcohol or soda at all and stick to water.
Consider your transportation needs. For many people who stay on site, Magical Express and Disney transportation are sufficient. Consider a cab for the "one trip" you wanted a car for, one round trip cab fare is generally cheaper than a week worth of car rental.
Don't fall victim to "everyone says we have to....." There are some really cool things that can be added onto a Disney trip - everything from the racetrack to character meals to Cirque to BBB....., and each of those things is on someone else's "must do" list. Your trip, your budget.