The Zombie Stack Exchanges That Just Won't Die
There are two primary contexts in which this might come up:
In either case, accusations of censorship, personal prejudice, or bad judgement are likely to come up. Please address these concerns.
Ideas:
I'm curious to read what else people post.
In addition to the above, is the material available through other sources such as Gutenberg, Internet Archive, LibriVox?
No library can collect in every area, so defining these using a well written Collection Policy or Guideline can assist when answering questions about collection areas and define the examples of statistics, condition, relevance, etc.
This type of public document is also a good way of handling accusations of censorship, prejudice, etc. by committing to and referencing things like the freedom from censorship policies relevant to your country's library organisation, eg: http://www.alia.org.au/policies/free.access.html, http://www.ala.org/offices/oif/statementspols/ftrstatement/freedomreadstatement As well as international statements like: http://archive.ifla.org/faife/policy/iflastat/iflastat.htm or http://www.un.org/en/documents/udhr/index.shtml
A collection policy can also be used to demonstrate that the library knows the rationale behind their purchasing, as well as acknowledge that our collections are complimented by strengths in other libraries.
Believe it or not, Awful Library Books can be a marvelous tool for making "OMG the library THROWS BOOKS AWAY!!!!one!" people rethink their stance.
I like to put together a display of books I want to weed (normally old, outdated, or in bad condition) and ask them if they want their kids using these books. Gets the point across most of the time.
Just a side note, if we weed books because we have doubles but they are still in good condition we put those out in a "free to take" bin. Some libraries give them to their friends group for book sales, but the one at my library doesn't want them.
Defending your weeding with a solid collection policy is one side of the coin, but you don't want to end up in a position of defending individual weeding decisions. You will set a pattern that never ends.
On the other side of the coin is building trust in your professionalism - show them you know what you are doing:
I agree that a solid collection development policy is a good place to start, but having a weeding policy is also important. Using the CREW Method, for example, you have a strong, consistent process in place. The act of reviewing & weeding feeds naturally into the act of acquisition (see page 10 of their manual). The weeding is dictated by a number of relevant factors, many of which folks list above: condition, circulation stats, relevancy, availability elsewhere (not just ILL but classics that can be found on Project Gutenberg). Using this method as the basis for your policy, you have strong arguments for not just weeding, but weeding with collection management in mind. So while you won't want to get into the nitty gritty of why each book was weeded, you do have those reasons, both as a whole and for each individual item.
Consider involving some of your teachers in selecting books they think don't belong in "their" specialty anymore. For example, I asked a teacher who used a lot of poetry to help weed the poetry and she was much more brutal than i would have been. Younger teachers especially understand the need for an attractive up to date collection.