I have not needed to do this for my camera, but I do this for my telescopes as a regular part of using them.
Dew wants to condesnse on the camera (and it ruins the shooting when it's condensing on your optics) because the camera is cool. If you can keep it warm (and it really just needs to be a degree or two above the ambient temperature) then the dew wont settle on it.
To deal with this on telescopes we use dew controllers and dew straps. The controller provides the power and sets the temperature, the strap wraps around the optics and you can think of it as a kind of electric blanket (because that's basically how it works -- except it's just a strap and not a full blanket.) The dew heater systems run on 12v system power -- which is fine when you're on a location and the camera is on a tripod -- not so fine when you're on the move and hand-holding the camera.
Telescope dew heater systems are sold by companies such as Kendrick, Astro-Zap, Dew-Buster, Dew-Not, Astro-Smart and many others.
But remember... the key to the whole thing is to just keep the gear a degree or two warmer than the ambient temps. Sometimes just keeping the camera close to your body -- under your jacket when not actively shooting -- can do that.
Tim Campbell
5D III, 5D IV, 60Da
