Compare ticket prices to see if it's better to get 10-day non-expiring and split it between 2-3 trips or just go with expiring-in-14-days, length-of-trip tickets. Remember it's $50 to add the same number of water park days to your trip as the length of your pass, regardless of whether you're buying a 3-day pass or a 10-day pass. 10 days at theme parks plus 10 visits to the water parks and Disney Quest can be a great money-stretcher, if you like those smaller attractions.
Besides the good suggestions others posted above, bring along healthy snacks to ward off the hungry, cranky grumbles... peanut butter crackers, granola bars, ziploc of nuts or trail mix, apples (~$2+ each, for fresh fruit or snacks in the parks!).
Buy a couple rain ponchos (2/$1) and fan spritzers at the Dollar Tree or other discount stores before you go if it's during summertime - they'll usually cost ~$10+ apiece in the parks. The 50 cent ponchos are thinner but you won't mind tossing them after one or two days' use. A fun thing at Dollar Tree: glow-in-the-dark bracelets, necklaces, etc. Bring those out while waiting for nighttime parades and fireworks - they come in cheap multi-packs and reduce the appeal of those expensive light-up souvenir toys.
Clothes vary widely in price from onsite to offsite, whether it's t-shirts, sweatshirts, shorts or swimsuits. So do toiletries and over-the-counter medications. Ask about clearance items in any
Disney store - the resort gift shops usually have some, as do the stores in DD. Better yet, stop at a local Orlando or Kissimmee
WalMart for your souvies - they stock a huge supply of Florida, Disney, Universal stuff. Just can't get the ride-specific souvies outside the parks.
If 2 people want a burger at a counter service restaurant, buy the double cheeseburger (2 patties, 1/4 lb each = 1/2 lb meat) which comes with a side (fries?) and pay for an extra bun (under $1). Altogether, it's much cheaper to share the 2 burgers & 1 side than to buy two burger meals. If you select one of the restaurants with a good fixin's bar, you'll have plenty of lettuce, tomato, mushrooms, onions, pickles, cheese sauce, to load up on the burgers. Don't overlook the rotisserie chicken family meals offered at some counter service places too. Under $20 to feed a family - very difficult to do with individual meal purchases in the parks. Ask for an extra cup and spoon when buying a double or triple scoop ice cream. Again, much cheaper to share these than to buy separate single scoops.
Lastly, if you have children who tan easily, look for Mickey-shaped stickers or get the small circle sticker dots in two sizes. Put them on the backs of their forearms or shoulders and show them how to leave a cute shape untanned in a couple days. If you're a little artistic and you have girls, get a little face sparkle (eyeshadow, make-up or facepaint) and put some on their cheeks and eyebrows, even in their hair or on their shoulders. Facepainting at the parks is quite elaborate and quite expensive! A (Halloween-type) eyepatch or bandana can be fun for a young pirate or cowboy too.
Children may already have these things at home but they're more fun at a theme park so bring them!
We found we saved money on souvenirs by giving each of the kids a $20 allotment at the start of the week and telling them that's all they could spend. All of them tended to wait a few days before finding what they "really" wanted and they didn't ask for more once it was spent.
The Hess gas station on property actually has decent gas prices (and decent convenience prices on milk, I've heard). The gas stations close to WalMart are pretty competitive for the area too. Some gas stations really rip off the tourists by charging obscenely high gas prices so look around a bit if the first prices you see seem high.
