I second everyone's suggestion to go early and take a break--wouldn't do it any other way.
Also, don't be afraid to slow down once in a while for the little moments. Some of our best memories are the little things that happen on the way to big things. One time, my DD ran into sleeping beauty when we were leaving through the world showcase exit. There was no one around, and she just sat on a bench and told Sleeping Beauty all about her trip. For her, it was so magical. On another trip, we were heading for test track when the kids got distracted by the fountain thing on the path between future world and world showcase. My youngest had never seen the "surprise fountains" (that what my kids call those things) and the older ones took him in. They spent the best 15 minutes of the trip laughing and getting wet--together. We never made it to test track that day, but I'll always remember the three of them playing in that fountain together.
Oh, and don't make your kids "pose" for all of the pictures. Candid ones are so much fun to look at later--you really see what they were thinking!
Have a great time!