Best time to buy Astros tickets?

I'm hoping to score some Astros tickets for fairly cheap, but unfortunately we've gotten too damn good for that to happen in a lot of cases. When would you guys suggest is the best time to score cheap tickets on StubHub or even just walking up to the box office?

I miss the days of $5 tickets after the 3rd inning when we were losing 100 games a year, made going to MMP so much easier.