Make sure you allow some time to relax - if you're overtired, you won't enjoy anything. Remember that you can't possibly see everything, so hit your favorites and enjoy the atmosphere. Don't get worked up if the family doesn't want to stick to the schedule. There is always room for changes. Remember you can cancel and ADR, play in a playground, spend a little more time at the pool, or ride Peter Pan twice and you will still have fun. You've done all the planning you need to do in order to have a great trip.
Make sure you enjoy the hotel. Service issues, mousekeeping problems and all the other things you read about are unlikely. Even if they happen, there's no place like a Disney resort. Don't panic if transportation takes a little longer than expected, just think of all of the rest of us who wish we were on a Disney bus and enjoy the sights and sounds around you. There is no such thing as a perfect vacation, but, if you don't worry yourself to death about the details, you'll have a very Disney vacation, which is the next best thing.
Don't nickel and dime yourself. You can't possibly squeeze every penny out of the
DDP or take advantage of every discount. Your time is worth more than the effort to figure out every detail.
Enjoy every family memory, take lots of pictures, let the someone else lead the way for a while, and take breaks from the heat. Watch the faces of your loved ones as they see things for the first time instead of looking around to figure out what's next. Hug a mouse, eat a Dole whip/Mickey bar/tonga toast/something sweet, see the castle at night, use fast pass, watch some of the live entertainment at World Showcase, watch a parade, hum along with the music, stand clear of the monorail doors, and have a wonderful, amazing trip.