I don't think there is really any rhyme or reason as to when prices go up or down. For the 2014 Feb/March trip to SXM I booked in May 2013. For the upcoming trip (2/26) I did not book until Nov. 2014 and paid slightly less than the previous year. I watched prices and they ran about $200 higher than what I last paid and then all of a sudden they dropped. I watched for a week or so as prices seem to bounce a little and then settle and made the decision.

I have also found that days of the week seem to matter little, at least in my case and from where I travel from. It has been reported that Tuesday at 2:00am (or something like that) is the "best" time to buy. I have never found that to be the case either.

Check prices often (but don't obsess) and when it gets close to a price you can live with, perhaps check more frequently but don't hesitate too much. Using a flight price tracking tool is a good idea also but I prefer to just check frequently directly with the airline.