I don't know if it is an option for you or not... but searching alternate cities is good way to compare fares. Such as flying out of Austin or San Antonio. And flying into Boston or Providence. Providence is less than an hour away. So if you are getting a rental car it wouldn't be a problem.
I fly to RI a lot. And sometimes Boston is a lot cheaper, other times Providence is. But I rarely pay less than $280 for a ticket. (Doesn't matter to me anymore. For the first time, as of November, an airline (AA) has a direct flight from DFW to PVD. I will pay anything not to waste time changing planes. Yippee
)
Edited to add: I wouldn't purchase more than 2 or 3 months in advance. Just start looking for exeptional deals 3 months in advance. If you start to get to 14 or 21 days out, go ahead and purchase.