Formal or informal?

みなさん、

Everyone who studies Japanese at uni learns formal Japanese and I remember being frustrated when I came across something written in informal Japanese because it was so different from the form of Japanese I had been studying.

Since university, I've been trying to learn and practice informal Japanese and I recently bought two books - Dirty Japanese and Making Out In Japanese II, (I already have book I). Ignoring all the vulgar expressions in the two books, it is interesting to see how informal Japanese is put together, and I have been using those books to try and understand it a little more than I do.

So, when I write to people, I know I should be writing in polite language, but I feel I want to practice informal Japanese. Sometimes, in films, there seems to be a fine line between the use of both forms. I had thought that if one was chatting away to ones friends, です wouldn't be used, but in a film I saw recently the character used informal language and then threw in a です as if to place emphasis on what he had just said. 

I am yet to find anything in a book or on the net explaining this, but I think I should try and practice informal Japanese. I just worry that if I focus on informal Japanese, I might forget certain aspects of formal Japanese. When I visited Japan recently, I remember practicing informal with my friend, but then accidentally using it to a shop assistant. I understand that one can use informal language, but I felt bad and a little impolite.

UPDATE:

Deva, the little evil cat, keeps telling me that I should never have attempted to write yesterday's post in Japanese. He has been shouting at me and telling me that I am hopeless and can't speak Japanese at all. I can defeat him! I can! 

Here's some ウルフルズ for you to enjoy!