The best way (and the least painful) how to expand your vocabulary is to talk with natives! Or go to places and just listen. You can learn slang or collocations and you don't even notice.

Dictionaries are useful too (but more painful). Learner's Dictionary, online part of the Encyclopædia Britannica, offers to the subscribers a daily portion of knowing consists of one English word. Here's the link: Word of the Day. There's also the possibility of testing your vocabulary skills, which can be both challenging and depressing.

Progressive people from Cambridge University Press came with apps to smart phones. Some of them are not for fee, but some of them are free. I would recommend for example Phrasalstein (I know, people from Cambridge are smart AND funny!). This app could help with phrasal verbs. It's also suitable for forthcoming Halloween.

My vocabulary has been expanding thanks to books. The most recent book I've been reading helps expand my empty Gaelic vocabulary. Thanks Diana Gabaldon, thanks a lot! Or how would the Highlanders say: Tapadh leat!