Disney has made this SO much easier with RIDER SWAP. Not all rides have this.(see list below) but most of the popular rides with height restrictions do!
How it works:
When getting on a ride that allows for rider swap, your entire group needs to go up to the first cast member present (so they know there really are other riders) and let him or her know that you'd like a Rider Swap pass. The people who are riding first will continue on and then the others will get the pass that works like a Fast Pass to use later. This pass does not have a specific time window like a Fast Pass, which is REALLY NICE! This pass can be used for up to 3 people.
I recommend having the first set of riders use a Fast Pass for the ride and then getting a rider swap for the rest of the group.
For example, if you were to go on the Seven Dwarfs Mine Train and are a family of 5...2 adults, a 9 year old, 5 year old and infant
1. Only the mother and 9 year old set up a Fast Pass for the ride
2. Have the entire family go up to ride and as for a Rider Switch Pass so the father can go on with with 5 year old after the first group.
3. The mother and 9 year old continue on while father hangs out with 5 year old and infant.
4. The father can then go on with the 5 year old and 9 year old using the Rider Swap (good for up to 3 people)
This way you maximize your Fast Pass options! This basically doubles your fast passes!
Rides with Rider Swap:
-Seven Dwarfs Mine Train, Must be 38 inches to ride
-Splash Mountain, Must be 40 inches to ride
-Big Thunder Mountain Railroad, Must be 40 inches to ride
-The Barnstormer, Must be 35 inches to ride
-Space Mountain, Must be 44 inches to ride
-Stitch's Great Escape, Must be 40 inches to ride
-Tomorrowland Speedway, Must be 52 inches to drive ALONE,
-Test Track, Must be 40 inches to ride
-Soarin', Must be 40 inches to ride
-Mission Space, Must be 44 inches to ride
-Star Tours, Must be 40 inches to ride
-Tower of Terror, Must be 40 inches to ride
-Rock 'n' Roller Coaster, Must be 48 inches to ride
-Expedition Everest, 44 inches to ride
-Kali River Rapid, 38 inches to ride
-Dinosaur, 40 inches to ride
-Primeval Whirl, 48 inches to ride