Here's what we do, especially at MK. We get a fast pass first thing in the parks, ride what we can until our time window opens up then get a second FP for something and ride our first FP ride. We do that as many times as we want to during the day.
I also don't sit down to a meal without getting a FP first. By the time you are done eating many times your FP time is ready.
Many times in the afternoon the FP times are for the evening hours so if we are going back to the resort to swim and come back to the park later we will get a FP. If we don't come back to the same park we can usually give it to someone who is going to that park.