Wasn't it Marshall McLuhan who said that we are "driving forward while looking in the rear-view mirror"? That's collection development. A lot of times you don't know that you need something until you need it -- you don't know you need it unless you lack it.
In an effort to stymie such minor informational droughts in a military library, I had started keeping a list of keywords from reference request forms. Over time, once scores of these forms accrued and I had lots and lots of keywords, I went back and attempted to "standardize" my keywords (derived from the words used on the form and from my memory of what the patron really needed -- these two things not always coninciding). I picked the most appropriate subject headings (used LOC subject headings), and made a new document with these.
Now I had a way to track the subjects (in a controlled vocabulary) that were sought in our library -- and I had a way to track the frequency in which they occurred over time. I kept it all arranged by date. I could track the kinds of topics that get asked for most often over weeks, months, or years.
Over time a pattern develops. You can begin to anticipate patron needs and make objective claims about needed resources (for budgetary or administrative reasons).
Of course, any body can get a sense of what'll be needed if you just get to know your patrons and listen to what they need for a few years... but a slightly more formal tool than "gut feeling" can help in a lot of cases, because things will inevitably slip your memory unless you make some kind of record. This is also helpful for librarians who don't plan on working in just one library for multiple years.
It's not hard to do (it just takes discipline to keep all this info together), and you don't even have to use subject headings or anything. But some kind of document similar to this is very useful.
It's like that car of McLuhan's. It's like being able to take an occasional glance out of the windshield -- highly desirable if you're on any kind of super-highway at all.
Listening: one of Terence McKenna's Palenque talks