I was in Jamaica last week and spent 4 days in Port Antonio. There are plenty of charter buses from Kingston (half way tree) which leave as soon as they fill up. The ride itself was cheap at 360J, and the journey through the mountains is beautiful. It does get stuffy at the beginning but once the bus starts moving, open the window and stick your head out and enjoy the breeze - nothing like it! People are very friendly and helpful and you can get off practically anywhere on the way - the couple next to me stopped the bus in the middle of nowhere and handed over their baby to somebody out of the window!
Beats spending 130$ on a taxi any day ;)
Of course, you have to be traveling light - there is a small luggage rack inside the bus but we just placed our backpacks on the floor (or lap sometimes).
From the Port Antonio station, plenty of route taxis to take you where you want to go.