There's no easy answer to this question, but tips, certainly.
Live in an English speaking country
This is the best tip. You'll be immersed in the language: people speaking it everywhere you go, newspapers, magazines, television, billboards, signs, and so on. I spent three years living in Germany and picked up more of the language "casually" than I ever did when I studied it at university.
Speak English as often as you can
If you're living in an English speaking country, then this should be no problem. The more you use a foreign language, the faster that language becomes your own.
The Internet is a vast repository of the English language: newspapers, books, articles. From the complete works of Shakespeare and Chaucer, to today's news about what's happening in your city or town.
Read the Internet for pleasure, too. I imagine you have a few hobbies outside of writing -- let's say, for example, you're into art history. There are hundreds of websites in English devoted to the history of art: museums, art galleries, art appreciation sites, personal blogs. Enjoy.
In addition to reading, listen. Many news websites have video feeds (e.g. CNN, BBC) and Youtube is swamped with movies and TV programs, and countless homemade videos of everything, from explaining how to defrost your car to packing a suitcase.
As a speaker of Spanish, you'll appreciate that the Spanish spoken in Madrid is different to that spoken in, say, Buenos Aires. Likewise, the English spoken in the United States is different to that spoken in England, or Australia, or India. So, if you're writing your novels in, say, London, it's probably not a great idea to bone up on the sentences of Australian English. Mate.
You're doing this already, and continue to do so: Read, read, read.
Copying sentences out is actually a great idea. Getting forensic with a language, although time consuming, will not be time wasted. The closer you study something, the more you will learn.
What if you're not living in an English speaking country?
It'll be harder, for sure. Use the Internet and read books. And seek out native English speakers in your city -- if you're really lucky, find a bunch of them who hang out together (maybe they get together to play sports, put on plays, or have a book group).