Just posting a lot on social media and news sites. I use Google Search, Google Translate, Mac's builtin dictionary, and Thesaurus.com (for synonyms/antonyms). Reading helps – you can "find" collections of ebooks for kindle/mobile quite easily (disclaimer: I don't support piracy, it's just that as a student I can't yet afford paying for 100% of what I consume).
I use google search a lot, when some sentence just doesn't sound right, I google it, if it has thousands or millions of results I accept it as right, if it has just some hundreds or less I rephrase it until I find a "popular" way of expressing what I want to say.