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I am looking for a way to prevent a security camera lens from fogging up. The camera is used outdoors and the temperature can get as low as -40°C. What kind of options do I have?

You have a couple of options:

One is to specify a an etched foil heating element that is ring shaped to just heat the outer diameter of the glass surface (approx. .125" wide annular ring).  This will reduce the aperture a little, but usually this is not a problem.  A surface mount sensor or thermostat could be incorporated for thermal control. 
 
Another option is specify winding a very fine wire heating element directly onto the glass lens with clear adhesive and insulate it with optically clear material.  This has the advantage of not reducing the aperture and allowing a more uniform and complete heated area, but in some applications the fine wire element in the viewing area  may be objectionable. 

Good luck!

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"A witty saying proves nothing."
-Voltaire

Condensation occurs when the glass is colder than the ambient air. As long as the lens is warmer, you won't get condensation. The outer shell of the lens is made out of aluminum, an excellent heat conductor. A simple heater around the outer body of the lens should be enough.

Indirect method - If you have a free source of dry Nitrogen (N2), you could purge the lens with heated N2.

Keith