Last time I arrived, I jumped on Juta Bus service at the airport.
It is a shared ride with maybe 6-8 folks on the bus with you.

The price was right and it did not take too long for us to be on our way.
(except for these 2 people that decided right before we left that they
had to get a few redstripes for the road)

I think I will try them again if they are still headed to the west coast.
Negril baby!