I actually like the way the FS Maui handles this kind of thing. You won't see the boorish behavior of people claiming lounge chairs by leaving their stuff on them and then disappearing for hours on end. Yes, you have to wait in line early if you want the prime spots but the attendants have every chair mapped out and know exactly which guest has claim to what chair. You only have a certain amount of time you can be gone before your chair gets given away. If all chairs are occupied, they'll take your name and call you when something opens up (instead of having to wait around).

At first I thought this kind of "system" sounded a bit rigid but it completely eliminates the problem the OP is describing.