You of course can still make a better deal with promotions, but slow travel and "buying two months in advance" don't mix well since it takes away the liberty to spontaneously change destinations or to leave a place if one does not like it (or to extend a stay).

But when one wants to fly next day in South America, in my experience the cheapest is to just buy at the airport, followed by online booking at the Spanish airline site, followed by the English site.