What can you do to prevent weakening eyesight?

Probably nothing. If by weakening eyesight you mean "presbyopia" which is changing focal length with aging, nothing. Gimmicks which have you exercise your eyes are of no value. It is caused by stiffening of the lens as you age. Other causes of decreasing eyesight, such as macular degeneration or retinopathy, can sometimes be treated by an ophthalmologist.