Let's talk about when you physically arrive at the park. In this case, I'm going to focus on Magic Kingdom
When you first arrive at the park, the FIRST thing I would do, is have a list of the top 2 rides you want to ride. In this case, let's say it's Space Mountain and Splash Mountain. I would head first to Space Mountain and get a FastPass, then head over to Splash Mountain and get in line.
The thing with FastPass, is that you can only have one at a time. It's tied to your ticket, you insert your ticket, it gives you back your ticket AND your fastpass.
The FastPass tells you what time to come back to the ride. So let's say you arrive at 10am, you go to Space Mountain, and the FastPass tells you to come back at 4:30 to 5:15. Now, you can go wait in the 30+ minute wait for Splash Mountain, and maybe hit another ride or two over there, and still have the FastPass for the main attraction Space Mountain. You can walk anywhere in Magic Kingdom in about 10 to 15 minutes. You can easily cross the whole park in 15 minutes, from one end to the other.
At Epcot, I would get a FastPass for Test Track, then head over and get in line for Soarin'
Those are the two main rides at Epcot. After Soarin, you can ride Nemo, then Spaceship Earth (the big globe), and maybe even head over and ride Mexico while you wait for your FastPass time for Test Track.
At Disney, you are always up against the clock. So planning your day is crucial towards having the best time