Clothing: About the only critical variable here is the weather you expect to encounter. It is a good idea to anticipate the worst possible conditions you might experience. Many race websites will give you the average conditions on race date. Some races list the conditions on race day for past years. The wisest thing to do is look at on the internet and find what range of conditions you might expect at the race location. Check out the Record High and Record Low for the days around the race date. This can be a little scary sometimes. I read an article in Ultrarunning Magazine last night about a 100 mile race in Florida this January. The temperatures at the start were in the 60s but during the race a front came through and dropped the overnight lows to 32 deg. That is pretty cold for Florida.
Next I will talk about layering, socks, shorts, and so on.