Same as at home. Check your bill, some places add a 10% service charge - if that's the case I just tip an extra 5-10% to get to the 15-20% I normally would tip. At a bar I do as I do at home also, $1-2 a round or something like that. Obviously it depends also on the service I got. I don't hold a long wait for food against the waiter - food is made to order almost everywhere in JA and takes quite awhile, but a friendly, helpful person is always going to get a better tip from me than someone who is just doing the minimum.

One tip when ordering - for most Jamaicans English is a second language (learned in school but rarely spoken at home or to other Jamaicans) so speak slowly and clearly

$US or $JA tips doesn't seem to matter much. Some say they prefer US$ because if they hold onto it for awhile it will likely get more valuable vs JA$, but most people will be changing the US$ to JA$ to spend it themselves.