I started to study English seriously when I was in the 3rd grade of the university. By then, I was just an average guy with spanish accent so people were like "he's English is weird". But now, 6 years later (scary... time flies!!), many people I meet recently tells me that my English is good.
Anyways, what I realized recently about language skills, is that you will always have an issue. Once you get to used to the language, when you can use it daily with not much stress, you will always reach to those issues that you will stuck in somewhere for a long time or sometimes forever. Don't you have those people surrounding you that never improve their grammar or their pronunciation? That's what I am talking about.
I just realized that once you get to used to use the language at a certain comfortable point, you suddenly become blind and you never wanna look back what is better to improve next. I think that is a problem if you are a business person, because we often look down unconsciously when you receive a terrible (I mean REALLY terrible) grammar mistakes.
I was point out that my vocabulary is poor recently from my staff. I knew that but honestly I was kind of satisfied with my English skills, so I felt shock. (I know I suck at English, but you don't feel like it when you are surrounded by non native English speakers.) But then I realized that if she didn't point me out, or if I didn't ask her about my week point, I had the possibility to never improve my English again.
Let's find someone who is better than you, and ask them always what you need to improve, what would be effective for your improvement, not only in language but also in sports, study in general, business, etc etc.
Then you can come up for the solution easily. For me, I decided to read an article of news in English everyday. Hope this lasts long!!
Kenta