Having had many years exerience, and flying to South America in the winters, there are several ways to get cheap flights

last minute
block bookings
registering with airlines

cheap flight provider

the list goes on. BE CAREFUL FOR HIDDEN CHARGES ( an amusing take on this by FA http://www.youtube.com/watch?v=ZAg0lUYHHFc )

Personally, I find a report like this useful :
go to http://bit.ly/bsZZG0

On another note, I found that being part of a club, both got me incredible deals, and paid me into the bargain :)
contact me on psmillion@yahoo.com - with country, types of holidays, and can point you in the right direction.