Remember that there is NO WAY to do it all. You'll be back. Relax, take time to just sit and enjoy the magic. Don't feel like you always have to go go go.
Take lots of pictures of your children (if you have any traveling with you, whether they are your own, or your extended families). Some of my most cherished pictures from Disney are of my neice's face lighting up when I took her when she was 4. Remember to use all cautions when traveling with film. Some airport security equiptment can ruin it.
BUT don't spend as much time behind the camera as you do just looking around. Get a cookie and a drink on main street and just sit and look at the castle for a bit. Take in a game of checkers on Tom Sawyer Island. Sit in a rocking chair in Liberty Sq.
USE FAST PASS. You'll be glad you did. Take some time before you get to the park to understand how it works and which rides use it.
Don't feel silly hugging the characters or getting excited to see Mickey Mouse. We all do, we're all 5 years old when we are in WDW.
Get a character meal at least once. Use PS whenever possible if you haven't set any up already.
Indulge in a Mickey Ice Cream Bar or two.
Strike up conversations with people; it's amazing how friendly 99% of the people in WDW are. Esp. interesting chatting with the CMs at the World Showcase about their home countries. Learn how to say Hello in each language.
With few exceptions, most of the stuff for sale in the shops, is for sale in all the shops. I find it best to save shopping till towards the end of my trip, when I've had a chance to look around and really decide.
ASk CMs to take pics of your whole group. They are more than happy to.
Bring a small notebook to jot down highlights of your trip to later turn into a trip report on the DIS!!
Bring comfortable shoes, and extra socks. Bring a rain pocho or umbrella, just in case. But don't leave the park if it rains. Duck into an indoor attraction for a while and be amazed at how thinned out the crowds are when it stops.
Most of all, relax, do not stress out, remember you WILL go back and it WILL be there waiting and HAVE AN EXCELLENT TRIP!