As Greg noted, the lenses have built in slots to allow for venting and drainage. Unfortunately, they don't have one way check valves to keep stuff out like many automotive lighting modules have.
You don't necessarily want to seal the light completely, but you could try a fine bead of clear silicone sealer between the lip of the housing and the lens, leaving a small unsealed area at the bottom of the lens for proper venting and drainage. I've done it on a couple bikes, and other than the fact that it makes removing the lens a little tougher it seems to greatly reduce the amount of moisture in the light. One other area to check is where the wiring enters the housing. The older setup on my SEEG wasn't sealed from the factory and water easily ran down the wires into the back side of the housing. A little clear silicone put a stop to that.
Thoroughly clean the bulb socket and contacts, apply a generous coating of dielectric grease with a new bulb, and try the small bead of silicone on the lens to housing joint.
Jerry
edit: I've got to type faster and quit letting things distract me before hitting the post button. 2lane explained the back side leak very well, and it is actually the more important of the two leak paths. The socket and reflector are actually sealed to the housing so water that gets in the back with the wiring and contacts doesn't drain out the lens, it sits in there and leads to corrosion.