My suggested itineraries will allow you to cover most of the ground in each park as efficiently as possible. Note, though, that using FASTPASS may require you to double back to a land you've already covered.
There are a ton of ways to see the parks. Time and budget permitting, it's often better to do it in limited doses -- where you spend 2 or more days in a park at a casual pace. My suggested itineraries are options for those on a tighter schedule, organized to get the most out of the least amount of time. Where appropriate, I break things into one game plan for families with kids and another for teens and adults. With few exceptions (noted later), Disney World doesn't have enough true stomach-turning thrill rides to warrant a special itinerary for teens or take-no-prisoners adults. Frankly, the only Orlando park in that class is Universal's Islands of Adventure.
Create an Itinerary for Each Day -- Read the Walt Disney World Vacations brochure and the detailed descriptions in this guide, and then create your own "must-see" list, including all the shows and attractions that you absolutely have to experience. After that, you can sort out just where to go, when to go, and what you would like to do while you're there.
At the same time, consider your loyalties. My younger kids could spend all day in Tomorrowland spinning around like space rangers with Woody and Buzz Lightyear, but touring Fantasyland is of far less interest to them. Put the ride featuring your favorite character, or theirs, at the top of your list. Sketch out a daily itinerary that includes your must-see attractions and shows; it's almost certain to change once you get to the parks, but will at least provide you with a good starting point. With a plan in mind and a map in hand (park maps can be found in this guide, but be sure to grab the free maps distributed as you enter each theme park), touring the parks will be that much easier. Understand that rides and exhibits nearest an entrance are usually the busiest when the gates open because a lot of people visit the first thing they see, even if the more popular attractions tend to be found deeper into the park.
I repeat this advice: Schedule sit-down shows, recreational activities (a boat ride or a refreshing swim late in the afternoon), and at least some unhurried meals where time permits. This will save you from exhaustion and aggravation. The suggested itineraries allow you to see a great deal of the parks as efficiently as possible. If you have the luxury of a multiday pass, you can divide and conquer at a slower pace and can even repeat some favorites.