Check what days the parks will be open and plan out what you are going to do each day. Sometimes they have parties for an extra fee at Magic Kingdom and it closes at 7:00 for everybody else. The Universal parks don't stay open late, Epcot always closes at 9:00. You don't want to be surprised if you go to the park on the day it's going to close early. You also don't want to miss out if a park that usually closes early stays open late.
NEVER go on Spring Break, it's too crowded and the first line you'll be in is a line to walk to another line.
Take a travel blanket for the plane, even in the summer they can be cold. Also it's good for the hotel room.
You can take your own water and snacks into Disney and Universal parks. I strongly suggest this is as food is pricey. If you have a fridge in your room, freeze the water bottle so it stays cool all day. Any of the drink stands will give you ice water for free.
Wear comfy shoes - you're going to walk a lot, we once walked 12 miles in a day. Croc's are good for the plane. They are also good if it's going to rain during the day as your feet won't stay wet.
Most likely you will go to one park each day, save your money and don't buy the park hopper passes. It takes a while to leave one park and get to another one so it's not worth it, especially getting into and out of Magic Kingdom.
Check the cost of annual passes, if you go 2-3 days a year to Universal or 9 days a year to Disney it's worth it to get the annual pass.
At Universal, don't buy a 2 park pass if you are going to spend all day in one park. We saw some people who bought a 2 park pass and spent all day in Universal Studios, by the time they got to Island's of Adventure it had closed. These parks don't usually stay open late.
If you go in the winter, bring fuzzy clothes for the evening. You can rent lockers to hold them in the day.
Take a little backpack or totebag in your luggage. You can keep just what you need for the plane ride in that and put your carry on in the overhead bin. Sometimes the plane is full and they make you check your carry on. This way you can carry on just what you need.
Don't bring a lot of extra stuff for the hotel. People pretty much go to a park all day, come back to the room, shower and sleep.
Get some glow sticks at the Dollar Store and for your family to wear after it's dark. If everybody in the family has the same color glow stick bracelet or necklace it makes them easy to find.
At Universal they make you check your backpacks on all the good rides. Anything other than a pair of glasses or a hat has to go in the lockers. If it doesn't fit in your pocket it can't go on the ride. The lockers are free for a half hour, sometimes longer if the wait for the ride is long, but it's a hassle getting your stuff in and out, if the lines are short and you want to go on the ride again. It also gets hard to remember your number after you've had a bunch of different lockers.
At Disney you can take small backpacks on rides, they have lockers at the front of the park for a fee.
Here's a picture of how the My Disney Experience App Works
Update Jan 2014 - coming soon, read all about the Disney Fast Pass Plus
