A good tip is to send only one member of your party to get FASTPASSes for everyone in the group. There's no need for everyone to trek through the park just to get FASTPASSes, and this cuts down on the amount of walking everyone has to do.
I'm not sure the "always hold a FASTPASS" idea is right for everyone. Using FASTPASS requires two trips to the attraction - one to get the FASTPASS and one to use it. During busier times of the year, the FASTPASS return time is generally so much later in the day that it is unlikely you'll be in the same area as the attraction when the FASTPASS time window opens. That guarantees at least one transition between lands per FASTPASS.
If you're doing a somewhat systematic tour of the Magic Kingdom, you'll either be in Tomorrowland or Fantasyland early in the morning, and typically in Frontierland, Adventureland or Liberty Square around mid-day. So if you picked up a FASTPASS in the morning in Fantasyland or Tomorrowland, the chances are good you'll be quite a distance away when that FP becomes valid. (Touring Adventureland while waiting for a FASTPASS in Frontierland, however, is a good strategy, as they're reasonably close together.)
Even if you decide the hike to the attraction is worthwhile, what do you do when you're done with the attraction? Hike back across the park to where you were prior to using the FASTPASS? This might be an appropriate thing to ask folks to do one, maybe two times per day, but I doubt most people would want to do it again and again throughout the day.
Finally, I don't think the "stay in one area while the FASTPASS becomes valid" is a generally good strategy. For one thing, getting a FASTPASS implies that the crowds are so large that waiting in line is bad. In Tomorrowland, for example, if Space Mountain's line is so bad that FASTPASS is required, what does that imply about the lines at the Speedway and Buzz? In Fantasyland, if you need a FASTPASS for Pooh, the odds are that the lines at Dumbo, Pan and PhilharMagic are significantly long, too. In many areas of the parks, there simply aren't enough viable options to spend time at. The best way to save time standing in line (which, presumably is what you're using FASTPASS for in the first place) is to move around the park.
If we make a couple of reasonable assumptions - like "don't leave one area of the park for another unless you can save 10 minutes standing in line" and we assume a reasonable walking speed - I think one can show that FASTPASS only makes sense in most cases when the posted wait time is more than 30 minutes. And there are lots of ways to avoid those kinds of waits without using FASTPASS.
Nice discussion. Hope this helps.
Len