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. On the other hand, elastic stockings reaching all the way up the thigh have been recommended as a preventive measure. Added to this is the simple matter of lifestyle. Drink lots of fluids - but not alcohol or caffeine. Keep your legs moving - walk around, stretch your legs every hour or two.