Speak as often as possible with native English speakers, listen carefully to sentence structure; and especially to intonation and diction.
Write often and only in English, ensure spelling and sentence structure is correct. Use the spelling that is appropriate. Don't be precious about spelling, if you live in the US, use US English, if you live in the UK, use English English. The same applies to grammar, there are differences between US grammar and UK grammar and is mainly related to the use of commas.