I know there are wind guards for motorcycles, specifically to protect the hands of riders. There is also a product called "hippo hands", which is a sort of insulated bag that goes over motorcycle handles and encloses the rider's hand completely, that really helps in cold weather. Unfortunately I was unable to find anything along these lines for bicycles. So I thought I'd make something myself: it might look goofy, but it does the job.
My office mates (some of whom also ride bikes to work, and suffer similarly from the chill) have been admiring my handiwork, so I thought I'd share the design. The tools and materials for this are basic, and you may already have them laying around the house (I did), but if not, they are cheap and easy to find.
Step 1: Tools and Materials
Two large plastic jugs. These can be from juice, iced tea (which is what I had -- my roommate likes the stuff, so I always have some sitting in the recycling bin), possibly even milk (though I thought clear ones would offer less distraction for my peripheral vision)
Two long zip ties (they have to go around your handle bars, all the cables, and have some slack for holding the jugs)
Two small screws for mounting
Duct tape, to cover the cut edges of the jug so they don't cut your hands
Exacto-knife or box cutter (optional, but very helpful for starting cuts in the plastic)
Hand-held hole punch