Cheap way : simple answer = No. Since the change in mileage accrual of FB in april 2009, all cheap fares earn 25 or 50% of distance flow (or 25 to 50% of 750 miles for European flights). So if you don't fly P/J class or full-fare Y class, it's very hard to qualify gold or plat
So the cheapest way to qualify gold/plat is by segment, even if it's time consuming. But you will be probably happy tospend more time in the air ?