Priceline has always worked well for me, but I don't know how they are for international flights (or if they will even do them?)

The trick with priceline is to find a good chunk of time to sit down and just mess with it. I got a cheap cheap CHEAP round trip ticket to Houston (from ORD) about a year ago that way - I decided to devote an afternoon to it and worked my way up from a ridiculously low price (my first bid was like $50) Keep going up by about $10 or $20 at a time, and I finally got my flight for I think $105 round trip. It takes a LOT of patience to sit there and wait for that confirm or deny email, though.