Sunblock, Very very important, helps to keep a nasty sunburn and a little refreshed. Also reapply at midday. Water mister are amazing (specially if you add ice to them).
Take your time going out of the park, specially at night, let the big crowds work their way and then on your own pace.
Comfortable shoes, (if their are new, use them prior to the trip, a lot) and socks...
If staying on property Extra Magic Hours!!!
If you are planning on buying souvenirs and send them, Also the stores can do that for you (don't do it if overseas).
Have lunch and dinner at odd times, like 11:00 am or 2:00 less crowd.
At MK see the rope drop ceremony, is a nice touch to start the day (I think all the parks do them. Except on EMH Mornings).
If driving, do a mix CD with your favorite Disney music, and do sing along (that helps the kids not to get bored).
Do extra activities if you can, Mini Golf, Disney Quest, Water Parks.
If with kids, make them do a journal of their trip. Also if possible give disposable cameras or cheap digital ones, so they can document their trip.
If going on a special occasion with kids, have them do t shirts for them and other members of the family.
Ask a Photopass cast member to take a group picture of your party.