First thing to check is battery cable connections, both at the battery and to the starter and ground. If a fully charged battery with nice clean connections still results in clicking, it's probably the solenoid. There is a disc in there that bridges some copper bolts to supply full voltage, and they can erode over time. On my forklift, and many other applications, the bolts were square head, and just turning them 180 degrees so the new part was in the contact area cured it.