2. Know What Time of Year You'll Be Visiting
Depending on which month you choose to visit Disney World, you'll find that the nightly room rates for all of their resorts swing up and down pretty drastically.
If you visit Disney's resort list page, click through to whichever resort strikes your fancy, and head over to the "Room Rates" tab, you should see a "View Rate Calendar" link for each type of room listed. On that page, you can check the rates that will occur over the next year.
This is a really useful way to get an idea of what a trip will cost you at any given time of the year.