I think there are two great ways to increase your "magic" on your vacation: 1) set expectations and 2) don't run the kids ragged. Okay, the third was already mentioned - plan ahead to avoid long waits for rides and meals.
1) As far as expectations, I started a month ahead of time explaining about the lines at Disney, how they may have to wait in lines, that sometimes when they want to do something we may not be able to do it right away because the line is long and we can do it later or the next visit when the line is short, etc. etc. etc. This worked really well, kids (mine are 5 and 7) really listen and understand when these things are explained throroughly and calmly ahead of time. If they are already begging to go on a ride right next to them it's too late to rationalize. I even spent time explaining the airport, security lines, airplane ride, ME, etc - any situation where they could be in line or waiting, I prepared them ahead of time.
2) We would hit the parks early and hard and get in alot of great rides, but as soon as the kids got tired and wanted to go to the hotel pool, we went. Kids need unstructured, fun time where they can just be kids. They were exhausted by 7pm, and although dh and I wanted to return to the parks at least one night for one of the shows, we just skipped it. Getting lots of sleep kept the kids in a better mood all day.
Hope this helps.