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. The factory installed foam also came loose after a number of years of use. I've seen a tray advertised in one of those gadget type catalogs that fits under the tank and has a tube that drains accumulated condensation into a container that sits on the floor under the toilet.
Water drains into two cups that are easily removed for emptying.
Some people claim that using a furry tank cover on the tank helps to prevent the condensation from dripping off the tank. I can't vouch for this however. When I tried this, it didn't work, because the hot water line was usually not hot when you first flushed the toilet. By the time the hot water got up to the area of the toilet, the toilet was finished flushing. This interesting fact was just brought to my attention a few weeks ago.