There's a lot of misunderstanding how Fastpass works. So let's start with the most important points.
Fastpass is free with park ticket!
First of all, Fastpass is free to anyone holding a Disney parks admittance ticket. So if you've got your ticket to enter Walt Disney World (or Disneyland), then you've got your ticket to use Fastpass.
Our best advice is if you are going during a busy time? Use early entry if staying on site and change to another park when that park gets to crowded for your liking. Don't leave before using the Fastpasses you have collected throughout the day. Make sure you get the next Fastpass when it is available. Send older kids with everyone's passes to get them and meet you somewhere. Always go back to the hotel mid day to cool off in the pool and relax for the late night hours. Get plenty of water during the day, which you can bring in....water is expensive at WDW!!
In order to use it, all you need to do is place your Disney park ticket into the Fastpass machine, where available and it will then give you a ticket with a return time for that attraction.
You then return to that attraction anytime during the "return time" or later, so you don't have to race back to get on the ride the moment the Fastpass becomes valid. As long as you return after the first time listed n the Fastpass you can get in line. It is a misconception that you must ride within the hour stated.
Once your fastpass return time has rolled around, all you have to do is return to the attraction, get in the fastpass line and hand it over to the Disney cast member. You might wait a bit before riding, but usually it is no more than 10 minutes.
Note the time indicated for getting your next fastpass
How often can you get a fastpass?
When Disney first started the fastpass system, you could only hold one fastpass at a time, but they've since relaxed the rules slightly. Now, you can get your next fastpass either at the start of the "return time" for your current pass or two hours after the one you are holding was issued.
Because you will generally be holding only one fastpass at a time, it makes sense to have an attack plan for the attractions where you want to use it. Fastpass attractions tend to be spread throughout the park to prevent bottlenecks in one area of the theme park, so you'll want to plan a route that keeps you from criss-crossing the park all day long.
Arrive early for popular attractions
It's important to go and get your fastpass early for the popular rides on busy days. Otherwise you may find yourself with a fastpass that isn't good until late in the evening, after you've long since left the park. So, it is always best to get to the park early, get the fastpass to the popular rides first and then when it is busy in the afternoon, use up the fastpasses while everyone waits.
On the busiest days, some attractions will run out of fastpasses very early in the day. This is common at Animal Kingdom's Everest roller-coaster and Disney-MGM's Tower of Terror, and the ever popular Toy Story Mania at the Studios. So get your fastpass early for these attractions if you're at Disney World in the summer or on a holiday weekend.
Fastpass isn't available at every attraction in the Walt Disney theme parks, but it is usually for many of the most popular rides, including: Space Mountain, Splash Mountain and Haunted Mansion at Magic Kingdom; Soarin', Mission Space and Test Track at Epcot; Rock-n-Roll Roller Coaster, Toy Story Mania and Tower of Terror at Disney-MGM Studios; and Kali River Rapids, Dinosaur and Expedition Everest at Animal Kingdom.