Use fenders and the bike stays a lot cleaner.
Clean and wax the frame with car wax.
wax or grease any exposed cables.
Grease the seatpost and stem (only threaded, not threadless types) where they are inserted into the frame tubes, to avoid seizing in place.
Keep the chain lubed, a wax like White Lightening is good in temperate rain.
If the frame is steel, squirt some WD40 into the small holes in the rear stays.

Apart from that, I dont worry. My steel bike was old when I got it and has seen 4 wet coastal winters with no real harm.