If airlines need to be held in any way responsible, perhaps it is only if they make a habit of keeping the 'Fasten Seat Belt' light on for the convenience of the crew rather than for actual passenger safety. Passengers should drink plenty of fluids. Smoking, which damages the blood vessels, should be avoided. So should alcohol and caffeine, which tend to be dehydrating. Sitting in an aisle seat is helpful, because it provides greater legroom than an inside seat.
Passengers can also exercise in their seats.
Whether aspirin helps is debatable. It does, indeed, help to prevent the clumping of platelets, one of the steps in forming blood clots. For those who tolerate aspirin, a half tablet taken two days prior to flight and another on the day of flight may at least be harmless. Clothing makes a difference. Loose-fitting clothes are an advantage.