I keep a reflective emergency blanket in my seat pack. It stays there all the time, right next to my spare tube, because even when temperatures are in the 50s or 60s, hypothermia is possible if I should have a crash or a mechanical.

I agree with Seajay, whether it's desirable or even possible to stay dry depends on the kind of ride. If you're heading somewhere you won't be able to change clothes, then dress to stay dry and scale down your exertion to avoid sweating. If you'll be riding hard and/or long, wool and stowed outerwear are the best way to be sure you stay warm if you should have to stop.