Watch a lot of TVB dramas and HK movies and repeat any phrases you find interesting or useful. If you still live with your parents, communicate with them even if they tell you that you suck. Yes, my mother calls me a banana all the time. >.<
And going to Chinatown is useful too. I'm serious.
And since you can speak bits of Cantonese, I'm assuming you pretty much understand what people are saying to you in Canatonese. If that is the case, speaking conversational Cantonese is only a matter of listeesning and practicing (yes, with your parents or a Canto friend). If possible, find an iTalki friend.
Good luck in HK. I wish I could return to the motherland. haha :p But that would cost me too much money.