Typically, for US and European airlines mid-December to mid-January is high season on flights to Brazil. When I was based in Europe, I would typically book my flights ~11 months in advance. Your only hope of seeing ticket prices drop around this time of the year is waiting till a month before your desired dates, in the hope sales have been especially low. But it's a tough gamble.
Otherwise keep an eye out on the airlines which operate single-aisle planes with a stopover (Copa Air, Avianca) which tend to be much cheaper than direct flights.