Research where to stay and decide whether you want to stay offsite or onsite (in a Disney Resort hotel). Once you've decided, book as early as possible to take advantage of any discounts that may be available.
If you plan to stay onsite and will only be arriving late in the day, it is cheaper to stay your first night in a cheaper offsite hotel, maybe near the airport. Then transfer to your main hotel early the following day.
Do plenty research before you go, online and read guidebooks. Planning is the key to having a stress free and enjoyable WDW vacation. If you learn as much as you can about the parks and attractions, the best times to visit, where to dine, etc, you'll have a much better time than the family that didn't do any planning.
Involve all of the family, even the smallest members, in planning for the trip. Let everyone decide which attractions they want to do, and where they'd like to eat.
Make dining reservations in advance if you want to eat at any of the sit down restaurants. This is especially important for character meals and the more popular restaurants. Reservations can be got for a lot of places once you arrive, but since the Dining Plan was introduced, a lot of restaurants are getting booked up further in advance.
Get everyone in the Disney mood! Have a Disney night each week when you can watch a Disney movie, sit around and plan what you want to do each day, or even have your own Hidden Mickey hunt.
Send for the free WDW Vacation Planning DVD - USA | UK.
Start collecting small change in a coin jar to use for spending money.
Buy Disney Dollars from the Disney Store, and give them to the kids for doing chores around the house.
Make a countdown wallchart, or a paper chain and remove one of the links each day and you'll see how long it is until you leave.