as everyone has mentioned, you need to immerse yourself with it in terms of who you socialize with. also the media you are immersed in (tv, movies, music)
Search - CantoDict
this site i like. has trad and simplified. has slang and lots of common phrases. and has spoken chinese in written form...
the tricky thing is with learning cantonese is that (and I think is part of why Rosetta Stone does not offer it, apart from a larger customer base):
classical Chinese writing (like what you would see in a newspaper and books) is not how people would speak word for word when it is person to person communication
ie 我是香港人 (classical)
vs
我係香港人 (spoken)
Think of it as your daily use of English. And compare it to that of your usage of English if you were to submit a paper for a university course. Only, the difference in structure and vocab are even more different