I subscribe to Going (formerly Scott's Cheap Flights). They have people and programs that scour the internet for discount and "mistake" fares, and email their paying customers.
They recommend booking
2-8 months in advance for international flights (calling it the Goldilocks Window).
FWIW, I consider the expense of Going well worth it. Subscribers can select which departure airports they want to have info on. I've seen some very tempting fares to every part of the globe, including SXM.
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