Drink water instead of soda. You can get a cup of ice water for free with your meal, and it will save you about $2.50 per meal.
Eat most meals at counter service restaurants so you don't have to budget extra for tipping.
Time your meals so you can get by with two meals a day, and a snack. Quick snack for breakfast, early lunch, and then dinner, or a large breakfast and then early dinner with a snack later in the evening, etc.
Order side dishes instead of entrees for your meal. For example at Yak & Yeti CS in Animal Kingdom you could get an order of Chicken Fried Rice and an Egg Roll for around $6.50, at Cosmic Ray's in Magic Kingdom you can get an order of chili cheese fries for less than $5.00 or at Boulangerie Patisserie in France at Epcot you can get a ham & cheese croissant, quiche, or sandwich for around $5.00.
Also, if you don't want a combo meal, order just the sandwich. It will be a bit cheaper (even if it isn't listed on the menu).