Having had both my brother and father suffer from blood clots, their doctor told them that whenever they go on a flight longer than about 2hrs, they should take aspirin.
Every long-haul flight we go on now, we all take 1 aspirin the day before the flight, 2 the day of the flight, and one the day after.
Hadn't really thought about it at work as most flights are around 3 hrs, but maybe I'll start taking it a bit more regularly.