|MadSci Network: Biochemistry|
the reason why tatoos fade away is simply that you are a living being! Your body replaces dead cells with new ones, transports a lot of substances it needs (oxygen, water, vitamins, ...) to the places where they are needed and transports the "rubbish" as for example CO2 out. So the parts of your skin where the colour of your tatoo is placed will be replaced and in addition they become "diluted" or dispersed over a greater area with time. Because of this explanation you see that there is (probably) nothing you can do to prevent it from happening!
Try the links in the MadSci Library for more information on Biochemistry.