I was using Chawla http://www.chawlatravel.com/ and www.bangkoktickets.com (better online interface) in the past. Good service (deliered tickets to the hotel in BKK), everything worked fine.
Since i need to go to Asia every few month i am buying flexible Round Trips there and just change the return to whatever i need.
Some of the very low fares are valid for less than a year, but you cann allways find good fares which are valid for a year and allow free changes of the return or changes for a small fee.