[QUOTE=Fish]
Disclaimer: I’m not a Hollywood insider, just someone who watches the commentary on every DVD I get. 
I believe it is generally considered cheaper to destroy and rebuild most large sets than to store them. During the filming of the Star Wars prequels, for instance, they would film all of their Anakin’s House sequences together and then dismantle the set to make room for Naboo Interior (or whatever).
Peter Jackson made special mention of the Bag End set: rather than see it destroyed, he purchased it. (I believe he paid New Line for it.)
Specially made props and costumes will likely be stored, and sometimes used by other productions. For instance, I believe the TV show ‘Firefly’ re-used the army costumes from Starship Troopers.
Long-running projects like Star Trek and its various TV incarnations are assigned a studio (or two). Their sets are made in pieces. The Enterprise bridge was made in a pie-shape so bits could be removed to make way for a camera. I don’t know about the hallways, but they could easily have been modular pieces re-assembled in different configurations depending on need.
Trek’s endlessly reused cave set was pretty much the same cave, every time, with different lighting and a few new rocks and set dressing here and there. That soundstage was dubbed “Planet Hell” by the cast.
The set design blueprints will be kept in case the set needs to be re-built. Sometimes this is easier said than done: the paint of the day is no longer available, or a color swatch is missing, a special piece of set dressing was missing, or — in the case of the Trek original TV set — the units of scale were not mentioned on the print.
[/QUOTE]
A number of years ago a friend of mine and I visited the Hollywood Entertainment Museum (which is actually worth a visit anytime you’re in that neighborhood, by the way), and they had a special exhibit of props and sets from Star Trek productions. There were about 20-25 feet of TNG hallway set (where we found that the captions on all the little panels, nearly invisible in the series, read things like “55,000 light-years per nanosecond isn’t just a good idea…it’s the LAW!”) which apparently was the whole thing. That’s all they ever used.
The Cheers set has taken up permanent residence there as well, and you can read all the crap that George Wendt and John Ratzenberger carved into the counter over the years.