Thesaurus.
Study linguistics.
Study English language. [Or whatever the target language is]
Those aside, reading high brow, or formal articles/publications/literature will broaden your vocabulary as well as your understanding of sentence structure.
Conversely, I often find myself unable to think of, or remember, local slang and idiophonic metaphors. I find local/regional slang very charming and it enables one's speech to be personal.
Curse my lack of street smart.
