I'm a FLR and have been to WDW at least twice a year (usually more) since birth. I'd guess around 100 times. There have been some great tips so far. But lets talk about my favorite subject. FOOD!
Bring those flavor packs for water with you. Poweraid, crystal light, kool-aid, and gatorade all make them. Take a fresh bottle of water with you everyday and drink at least 3 normal sized bottles per day. As in stay hydrated. I've seen at least a dozen people collapse from heat in WDW over the years. Usually on the backside of WS in Epcot - btw. This saves you money of buying drinks, and is better for your body than downing soda all day.
Eat breakfast in your room. Something small like one of those oatmeal cookie things by Quaker. Or if you don't want to bring anything with you get something small from the CS place at your resort. Like a parfait or cinnamon roll, etc. Also if you happen to be at either MK or Epcot stop by one of the bakeries there for that breakfast for the next morning.
Have an early CS lunch. Like right when they open at around 11am. The food will be it's freshest then, and hardley anyone will be in there. I often buy kids meals for myself at CS places when on my own. Usually more than enough food for me. And it's 4.50 instead of 8-15 bucks. Plus they often have healthier options, but in recent years the Adult platter have gotten better. Or I share an adult size meal with others.
I always get my ADRs for TS meals at around 3 or 4. This does a few things, it's an "off time" for the restaurant. So you will have less of a wait time once there and will most likely get your ADR when you call. Plus this is the HOTTEST and BUSIEST time of the day in the parks. You'll be sitting pretty in a nice ACed TS restaurant.
Then at the end of the night grab a spot for the fireworks (or whatever) and have someone else go grab a snack for everyone. Waiting 30 minutes for illuminations with a chocolate crepe and a GM slush does make it a whole lot easier.