For the English-as-a-second-language folks, what I found crucial was interest. When I attended English classes - since primary school, as my school was bilingual - I could clearly notice a difference in every aspect (vocabulary, grammar, etc) between the ones who didn't really care, and the ones who did. (The school itself was good, and therefore, the bilingual capacity wasn't its only advantage).
I've had some classmates who evidently weren't interested, and - even when we reached the final stages of school - made mistakes that were too simple (I have seen a few write "I doesn't", and not on purpose) but I knew they didn't care about their English development, and that was reflected in their performance.
Something that caught my eye a few years ago, is that I have begun to read more books in English, and realized that my vocabulary was lackluster (or I wasn't satisfied with what I had). Therefore, I decided to - instead of guessing the words's meanings by context - search and store as much vocabulary as I could. After a few months, I could definitely perceive progress, amplifying my spectrum in order to be able to express myself more accurately and precisely. I am not there yet, but I have made progress.