Find examples of the type of writing you want to be good at in the future. They MUST be written by native speakers of English, who are writing for other native speakers of English, and not for students. Examples that are long and difficult are the best - you don't want a collection of interesting sentences in a Word document, you want whole articles that are many pages long. Read, read, read. Read some more - ten or twenty pages every day, or more if you can. Do not choose "safe" things that you are comfortable with; NO English textbooks or special ESL materials; only "real English for English people". Don't worry about understanding every word, or being able to write all the things that you see - just read read read.
Since you are interested in economics and finance, you should certainly try to get some famous English magazines such as the Wall Street Journal, The Economist, and any other good ones you have heard of.
It would be hard work, but one excellent way to practice is to copy (by hand, with a pen) the articles you are reading. Don't just copy the interesting parts - copy the whole thing. It takes a lot of time, but it might be worth it.