1. A computer-implemented method comprising: receiving a plurality of first search queries, each of which comprises one or more first query terms; constructing at least one association graph that includes at least a portion of the first query terms as vertices; receiving from a user a second search query comprising a plurality of second query terms; using the at least one association graph, identifying one or more suggested replacement terms for one or more of the second query terms; presenting the suggested replacement terms to the user; responsively to a selection of one of the suggested replacement terms by the user, substituting the selected suggested replacement term for the corresponding one of the second query terms, to generate a refined search query; and presenting search results to the user responsively to the refined search query.