My "strategy" is too plan a trip over a year in advance, may be 20 months or more. Then I keep track on the spend to ensure the voucher is triggered about 50 weeks before the planned return date. That way the voucher is triggered around the time the seats become available. There is no guaruntee seats will be available so no plan is foll proof, but so far I have managed redemptions in F to BKK and JNB/MRU (some of the hardest routes to get) for my required dates.