I'm a big fan of
http://FrugalTravelGuy.com, the blog by FlyerTalker
ingy Rick Ingersoll. He tells how he and his wife travel internationally for free or deep-cheap several times a year. I've met Rick and his wife, they are the real deal.
Be sure to look back at his blog posts from past weeks...the list is at the bottom right column.
He says the "law" at their house is, if a flight is over 3 hours, it's got to be in biz or first.
The Frugal Travel Guy book is inexpensive and full of great information.