At PLA this year, Joan Frye Williams and George Needham addressed the best ways to get community input (Tell Me Something I Don't Know: Meaningful Community Engagement). Using surveys was bottom of the list of choices - when you ask individuals for input, you get idiosyncratic information. Their suggestion, which would apply to collection development as well as any library service, is to talk to groups of people or individuals who represent groups of people. Instead of asking John Doe who happens to come into the library a lot what books he wants, ask the head of the local home-school association, the chairpersons of local HOAs, the local environmental group, area PTA presidents, the president of the Chamber of Commerce, area book clubs. These people can tell you what is on folks' minds. Individual library users, as Tatjana noted, will usually submit their personal choices through the library's request system.
The talk by Joan and George will be presented as a free webinar by Infopeople on June 12, 2012: http://infopeople.org/training/tell-me-something-i-don%E2%80%99t-know. I highly recommend it.