The answer to the title question is really simple. It´s proved that you can actually learn new words, slangs, expressions and even grammar, improve your pronunciation and your listening skills with pretty simple things such as reading books (or listening to the audio ones), magazines, websites, watching movies, series, tv channels, etc. In my case, I really love sitcoms, so it´s a real pleasure to watch them and write down all the new words. I watch them with the subtitles in the language I want to learn/ practice (Spanish and English for me). I have some notebooks I use only for this purpose and it really works!
I´m gonna give a few examples to ilustrate this. Once a student asked me how to say "meu cachorro solta muito pelo". After putting some thought into the sentence and almost giving up, I caught myself saying "my dog sheds a lot". I was amazed at my own answer! Another recent case was when a student asked how to say "cofrinho (da bunda)". I was a little faster to answer this one. Without hesitation I said "buttcrack"! I looked it up just to make sure and there it was! The other case was in Spanish. I was in my Spanish class and I wanted to say the word "audition", so I asked the teacher if the word "audición" was ok. He didn´t know, so he looked it up. Even being a native speaker of Spanish, he said he had never heard this word before. I told him I had heard it when I was watching "Friends" in Spanish. It´s surprising to see how this simple and yet extremelly pleasant technique works so well. I guess the knowledge is stored somewhere in your brain. It´s like watching tv while your´re falling asleep (or vice-versa) and you end up dreaming about the show or the movie. Or also when you listen to a song only once and you start singing it along the next time you listen to it again. I think this has a lot to do with language aquisition when we´re babies. We start speaking by repeating the words we hear most often. It´s amazing, but it´s true, so why not give it a chance?