Fast Passes are great! Especially if you use them right. For the more popular rides (that we may want to ride twice) we'll go there first (upon park opening) and get a FP and then ride it. This works really well for KS at AK, most of the time our FP time is up when we get off the first ride so we ride right away again (after getting another FP!)
If you are at a park (like MGM) with 2 FP rides located close together (like ToT and RnR) We'll get the FP for Tot, ride it, and before we ride again, we'll get our FP for RnR. Usually, the time has passed on our first FP that we can get our next one. (Hope I haven't confused anyone).
The more FP you can get early in the day, the better, since as others have said, they go early. The FP does state the time you can get another one on it. It is either the earliest time posted on your FP, or 2 hours later, whichever is soonest.
We used this strategy a couple weeks ago at DL, on a Sunday. We got there when it opened (contrary to what I've read, the park does not open the main gates early) It took about 30 min. to get into the park. (open at 8 a.m.) We went to Buzz, got a FP, rode it, went to SM, got a FP (the time frame was already up on the Buzz FP) rode it. Rode Astroblasters. Went and rode PoC (no FP) and then went back and rode Buzz. Had lunch, rode SM, etc.

We frequently used our FP past the time posted and it has never been a problem. As long as it past the first time posted, we've always been able to use it. So don't throw those early in the day FP away. Use 'em
