Identifying the Fog
The first step a to take in repairing a fogged camera lens is to identify the nature of the fog. What might appear to be fog might be a smudge inside the lens (which can come from previous repairs) or even fungus. If the fog is a smudge, a simple cleaning is in order; if the fog is fungus, you will need to kill the growth with a low-toxin agent. In many cases, vinegar is used. Clean the entirety of the lens in fungus cases, as it can spread and eventually corrode a large area of the lens.
Disassembling the Lens for Lens Cleaning
The first step taken in lens repair is to disassemble the lens. This is done with extreme care and precision, as the lens can be easily scratched, and the individual components themselves can be damaged if not taken out one at a time. With the lens disassembled, try to remove the fog (or smudge) first without a cleaning agent, then with water and last, with special lens cleaning agents. If none of these work to remove the fog, it is possible that an irrevocable damage has been done to the lens coating, which is extremely sensitive. If the fog or smudge has been cleaned from the lens, check the seals around the lens to make sure that no more moisture can enter or exit the lens. Also check the lens for cracks, which may also allow moisture inside the camera.
Removing the Fog by Drying
An alternative method that can be used to remove fog from a lens is to dry the camera out (this is treating literal fog, not the appearance of fog that is, in fact, a smudge). To do this, you would need to find an airtight container into which you can place the camera. The Classic Camera Repair Forums suggest an ammo container with a rubber seal as an airtight container. In addition to the camera, you would place a drying agent, such as silica gel, into the box as well. Given some time, all moisture will be sucked out of the camera, removing any and all fog from the camera lens.