Those with low blood pressure (sometimes caused by medications) can prevent feeling dizzy or light-headed on standing by taking the simple precaution of rocking the feet on the floor a few times before getting up - this stimulates the blood flow and increases the volume of oxygen to the brain.
Ankle rotations and foot flexions (good toes/naughty toes!) are also useful and easy seated exercises to do on long-haul flights to prevent deep vein thrombosis (blood clots brought about by poor circulation).
At home, these exercises can also be done whilst watching TV (during the ads, when you've got a couple of boring minutes to fill!) as an easy way to stimulate the circulation and ease ankle joint stiffness. To make it more interesting (and stimulate the brain at the same time!) try spelling your name backwards/upside down with each foot in turn, making sure to keep your knee still and make your ankle movements slow and smooth. Your feet will thank you for it :-)