Bring extra sunglasses, hats, socks and underwear. I usually go through double my allotment of socks and underwear while at WDW due to the heat and humidity...I'll change during the mid day break. I usually pack two full changes of clothes for each day. One to get all sweaty and sticky in during the day... and then a different change of clothes that I change in to right before dinner and wear at night. After showering and changing my clothes I usually get a second wind and am able to rush in to the parks again. If you are going for a long trip (more than a week) and you don't have 14 changes of under clothes to bring, or just don't want your suitcase bursting at the seams, then bring some purex all-in-one laundry sheets...detergent, softener and dryer sheet in one thing...just throw it in the washer and move it with the rest of the clothes in to the dryer. 24 loads of laundry supplies in a tiny space.
Extra sunscreen for sure! Bring twice as much as you think you'll need. Reapply liberally throughout the entire day. Especially the waterpark days.
Bring two COMFORTABLE pairs of shoes for each person and wear them on alternating days. Some people bring a pair of walking shoes and a pair of flip-flops. This will let your shoes dry out each day, and also prevent rubbing in the same spots and reduce the number of blisters. I would also search the disboards for some tips on foot comfort while at the parks. Many people suggest moleskin, socks of various materials, padded insoles, etc. There are entire teams of scientists on disboards devoted to this subject...My recommendation, only bring shoes that you can currently walk on at home for an entire day without any discomfort at all... because at WDW, with 9 times as much walking, those tiny little annoying spots of barely noticeable discomfort that you notice on your current shoes at home, will be screaming for you to end the torture with just 1/2 a day of walking in the parks.
Bring lots bandaids, and an anti-chaffing stick (or two). The stick will help prevent blisters, and the bandaids to cover up the blisters you might get.
have someone in your group get a waterproof wallet, lanyard, etc. for the water parks to hold room keys, money, etc. Don't forget the water shoes for the water parks. The pavement gets VERY hot for bare feet.
If you don't mind the taste of the tap water in Orlando, then you can bring one of those metal water bottles. I have mine with a Carabiner, so it can just clip to whatever. Then, just fill them up before you leave your room, and at water fountains in the parks. I think, if you ask at any of the food locations, they will give you a glass of water that is run through the soda fountain filter.
Finally, don't forget the chargers for your various portable electronics... cell phones, iPods, digital cameras, etc. Extra batteries for those devices if they take old-fashioned AA batteries. Extra memory cards for your digital camera, and maybe the cable and software you need to download your pictures to your laptop while you are there. I always take more pictures at WDW than I thought I would.
It makes it easier for the kids when they are done with clothes to throw them in the right basket in the first place and saves mommy sorting time on vacation 
