While the small but growing field of corpus-based work on metaphor offers numerous examples of fine-grained analyses of metaphor as it occurs in discourse, such work has generally been extremely narrow in focus, examining the distribution either of particular, searchable lexical items or of instantiations of a certain conceptual metaphor within a given discourse domain. Aiming for a broader approach, this paper addresses some of the methodological issues associated with using corpora as a tool for metaphor research, and proposes criteria for identifying metaphor in spontaneous discourse. Based on approximately 500 metaphorical utterances drawn from the Santa Barbara Corpus of Spoken American English (DuBois 2000; DuBois et. al. 2003), this study assesses the overall frequency of metaphor in discourse, as well as the relative frequencies with which various types of topics are referred to metaphorically, towards the goal of a better understanding of how and why speakers use metaphor in common language. It is discovered that the frequency of each of these categories of metaphor corresponds closely with its degree of concreteness, as verified independently with a survey of college students. In addition, this paper explores the role of metaphor as a discourse-level structuring tool. The overall methodology is offered as a model for future research.