Condensation on the outside of a toilet tank is the result of humid air in the room coming in contact with the cold tank. This will happen on a glass filled with a cold drink. Condensation on a toilet tank is most often a problem in the summer time in humid climates, but can also be a problem in the winter. In addition, houses on well water suffer a great deal from this problem. Houses built in the 70's are more air-tight and energy efficient, consequently humidity is easily trapped indoors.
Here are a few more causes/solutions:
Is the toilet leaking? A constant exchange of fresh cold water in the tank will cause condensation to form on the outside of the tank when the humidity is high in the bathroom. Check the toilet for leaks.
Is there enough ventilation in the bathroom? You may need to install an exhaust fan.
In winter, the condensation may be caused by using a humidifier on your heating system; if so, can the humidifier be adjusted?
Toilet tank liner kits are available to insulate the tank, check your local hardware store or home center. You must follow the directions carefully regarding cementing the foam to a clean, dry surface. Even then, the foam will probably loosen after a few years.
Drip trays advertised in one of those gadget type catalogs that fit under the tank. One type has sponges that absorb the condensation and the other has a tube that drains accumulated condensation into a container that sits on the floor under the toilet.