There is an excellent book called 'How to Fly Cheaper' by Hugo J. van Reisen that explains all these and other rules, including these where to buy your ticket cheaper (countries etc). A bit old now but still well worth a read for long haul ticketing.
His slagging of KLM is hilarious and worth it for that alone, but it's understandable since he's Dutch. (the book's in english tho).