I could have sworn that, years ago, I learned that the proper plural was "indices" and that "indexes" was a less-preferred form that was slowly gaining dominance.
Webster's New World, American Heritage, and Merriam-Webster all list "indexes" as the first choice for the plural. A Google search shows that users feel the same way, 75 million to 42 million in favor of "indexes."
So I pulled out my 1955 Oxford Universal Dictionary, certain I'd find evidence that the preference for "indexes" was a recent development. Nope again. This big dusty book also prefers "indexes."
Too bad. I like "indices." If it were preferred, it would be like a fun plot twist in the middle of written work -- something to keep you on your toes. But no.