Survey faculty and students: What do they want or want more of?
Perform a word analysis of faculty websites: Copy and paste from faculty websites the words that describe their research and teaching interests. Compare this with subject headings and/or Dewey classifications within the existing collection.
Consult with selectors for subjects that are related to the one for which you are responsible. Are these related collections strong in ways that supplement the subject you are trying to build? This should help in deciding priorities—e.g., if the political science collection is strong on Latin American politics, then the new Latin American line might temporarily leave politics as a lower priority than some other subject matter.