In consideration of the "Millions of thanks", I shall try :-)

In my opinion, if you have some working knowledge of English, the best way to improve is to use it ACTIVELY. Yes, I put the word "actively" in captical letters on purpose.

Try to speak to native speakers. I cherish the lunch time with my English-speaking coworkers and try to talk about various topics.

Try to write and rewrite. It helps a lot to identify and correct your errors. If you are not sure, ask for help.