Eat an aspirin, (or half of one) half an hour before you get on the plane (the timing isn't critical). If it's a long flight, have another perhaps four or five hours in. This will have an anticoagulant effect on the blood, greatly reducing the risk of deep vein thrombosis, also known as economy class syndrome. As already mentioned, moving your limbs throughout the flight also reduces the risk.

Don't listen to the in-flight movie/other provided entertainment, they charge a fortune for one of those little adaptor thingies that plugs into the armrest! Instead, try to bring some form of portable music player, and some good headphones (the big can ones).

Have a nice flight!