We learned by total immersion. There were months in which we wouldn't even see foreigners. During the off season we'd be up on Zion Hill or over at the Estate for weeks on end. Only time we'd go to Negril was to visit our Jamaican friends and family.
Miss G understands every word but doesn't speak with the accent. However, she uses the proper idiom. I on the other hand have always loved to mimic and so I worked on my accent, pronunciation, body language etc.
Remember that all Jamaican's speak two languages Patios and the Queen's English. I speak the Queens English in any formal or certain other situations depending on whom I'm conversing with.
Just about the only time I speak American is when talking to visitors