From 3/28/2004, a post that asks a question for which I still don't have a good answer:
How many times does a word or phrase need to be repeated in order to seem characteristic of a speaker or author? I think that the answer is "not very many times, maybe only once or twice, if the use in context is salient enough".
Ruminations on related issues can be found in "Strange Bookfellows" and "Captain Crunch among the Literati". And since this question tells us as much about the reader or listener as it does about the writer or speaker, we should also consider the curious case of the president's pronouns.