Preparation is everything. It's looks like this Wednesdays twilight cross race will be the first "cross" race of the season, locally anyway. By "cross" I mean wet and cold. But you don't have to be wet and cold while you're doing it.
There are a few things you can do to your bike to prep it for wet racing. Most importantly you'll want an aggressive set of tires. If you're using clinchers as I assume most of you are, I really fell in love with the Clement PDX tire last year. It's widely spaced tall knobs make for an awesome wet weather tire. I've also had really good success with the Michelin Mud 2 and it works really well in dry conditions too so it will do double duty for you. If you're running tubulars it's all about the Challenge Limus, if you're lucky you might be able to find the Limus in a clincher, but they're a rare find.
The other often overlooked item is your brake pads. Don't start the race with worn pads. Wet races wreak havoc on your brake pads. In both mud races I've done this year I've gone through two brand new sets of pads in each 1 hour race. Nothing is worse than grabbing that brake lever and having it pull all the way to the bar.
More is more. Don't be shy, overpack, you'll thank yourself after the race.
Towels, towels, towels: Bring a couple towels, some wash clothes and I like to have a thermos of warm water to use for cleaning up. Having the gear to clean up after the race makes racing in it much more enjoyable. The warm water is a welcome comfort when you're wet and cold.
Neoprene gloves are the cross racers best friend in cold, wet conditions. Think wet suit when you hear neoprene. One caution though, you'll want to bring a second set of gloves to warm-up in. Do NOT warm up in these gloves, save them for the race. Your hands sweat pretty a lot in them and your warm toasty warm up will turn into cold freezing hands while you wait for the start. My favorites are the Castelli Diluvio.
Embrocation! When it's cold and wet I go for Mad Alchemy's Medium embrocation. This warming paste will not only keep you bare legs warm but it also sheds water. Think of it as an Tiger Balm Vaseline mix, which I think is what I used before embrocation was readily available in the USA. When it's wet I like my legs to be bare. Leg or knee warmers only hold cold water against your skin making you even colder.
Eyewear: It's a fine line to walk. Sport glasses are great for the first couple laps when you're fighting for position and getting sprayed by the rider in front of you. But once those lenses are dirty you usually have to toss them to your pit crew. One trick of the trade is to treat your lenses with RainEx or Oakley's Hydrophobic lens conditioner. If it's raining heavily, this treatment is a must on the inside and outside of the lenses.
Helmet: Those of you lucky enough to have a Lazer helmet should look into getting one of their aero rain shells. The rain shell is a thin plastic cover that snaps over the outside of the helmet covering all the vents. It not only keeps you warmer but also keeps the cold rain water out.