Agreed. Oh, Marcos and Marcelo Pias, have already spilled the beans!
I am not that good at it, but I can say that variety is the spice of life
, we read some newspaper, novels, and listen people talking in interviews etc. Along with the watching of movies, clips, docs, interviews, what you have.
It helps to get a dictionary of idioms and expressions, but you can learn from sites. Provided that you get them from good the good ones, from experts etc. Word reference, Kudoz, Linguee, among others, could be cited.
Pratice makes perfect, it gets better with time and dedication. Believe it or not, we also sometimes get rusty as well, we learn and then we unlearn things (sentences/expressions etc). Then we try to say it in other ways, just like in Portuguese.
To the outsider (I mean, the reader now) it goes unnoticed.
Sometimes I get myself at lost for words, even in Portuguese (I remember the English expression, but not the Portuguese counterpart), then my friends fill in words. It´s the effect of having too much on one´s plate, and the proof that we aren´t computers to store all data, all the time.
The best medicine, is praticing. The second best is not getting overly anxious.