The most common cause of a dying jade plant is improper watering. Over-watering gives the roots more moisture than they can handle, causing them to rot.
Proper watering depends on the time of year for the jade plant. In the spring, summer and fall months, the plant requires its soil to be slightly moist all the time. This is accomplished by soaking the soil thoroughly and allowing excess liquid to drain through the pot's bottom holes. The jade plant takes a break from growing during the winter months, so it requires a lot less moisture. During winter, the plant should be watered with a spray bottle or by drizzling water from a small squirt bottle to avoid over watering. With proper care, the jade plant can grow to a height of up to two feet.