Here are a couple of my tips:
1. if you buy the refillable mugs sold in the Disney hotels, which my family always does, bring a small tube of dish soap with you to wash them out in the rooms-better than bar soap, and we get our money's worth drinking pop, tea, etc. with our mugs-plus, great souveniers for our family.
2. If you have a child who is unsure of some rides while you are in your planning stages, go online with them to check them out. My 7 year old Hates anything fast-so to show him that certain rides were not coasters, we went to website like this and others (including the disney website) that offer downloads or views of rides, and took a virtual tour-it really can put your mind and your child's at ease.
3. I know others have mentioned baggies full of snacks to take in the parks, but I always bring small tupperware style containers in our backpack to the parks. It keeps chips and other crushable snacks in their own containers without breaking-I pack one for each kid, since they like different snacks, and that way kids and adults have their own snack "package"....
4. I bring a water bottle (sport top) filled with ice for each of us-we use it for water, or after drinks or snacks or meals in the part, fill it up with leftover beverages, and that way they stay cool pretty much all day.