How can I improve my English?

I came to the States as a student and have been here for several years. My English is not too bad, but I am not really happy with the way it is. (It can always improve, you know.) I am specially interested in improving my ability of speaking. How would I improve my speaking abilities? To be more specific, how can I learn more idioms, new words and their usage? I understand that probably I won’t ever be able to speak like a native speaker, especially when it comes to pronunciation, but I would like to somehow be able to improve my level of communication. Any advice would be appreciated.