I actually disagree with the "book in the order they will run out" strategy" IF it sticks you with 90-minute waits in the early morning. The morning is the best time to get the "immediate" returns that can let you ride multiple FP rides in less than an hour, so I usually book those for the first couple of hours and don't book a "90 minute wait" one until at least 10:00.
Here are a couple of ways you end up with more than one FP at a time:
1. As mentioned by PPs, book ones for later in the day, and 90 minutes later, book a new one. So if you book a 6:00 return for RSR at 11:00 am, when you pull your next one 90 minutes later (at 12:30), look for something else with a 6:00 or later return time... and then 90 minutes later (at 2:00), look for something else with an evening return time... same thing at 3:30... and 5:00... in that case, you could take a break from 1-5 or something, and you'd come back with 5 evening FPs waiting to be used.
2. Book a FP at start of your FP return window when you haven't used the FP yet. We'll end up holding 3 at once a lot this way - say, at 9:20, you book Matterhorn with a 10:00-11:00 return time. At 10:00, you log in and book "It's a Small World" with a 10:10-11:10 return time. At 10:30 (30 minute minimum wait), log in and book Roger Rabbit with at 10:35-11:35 return time. Now you're holding 3 FPs for rides that are close together; you can walk over there and use all 3 (keep in mind they'll scan you in 5 minutes before and 15 minutes after the listed time).
**Note that alternatively in scenario 2, you could book that "It's a Small World" one at 10, walk over and use it as early as 10:05, and book a new one immediately. If you were in the area, that would be most efficient. If you're over at Haunted Mansion or something, it's nice to have three grouped together, waiting for you.
Hope that helps.