I second (or third) the drink advice. I don't want to drink when it's cold either and I can sure tell it at the end of the ride and the rest of the day.

I'm just trying out a wool base layer for the first time (SmartWool Women's Microweight Tee) and it's just wonderful! No, it doesn't give up your sweat like a synthetic will but it really absorbs sweat and doesn't feel cold or clammy like a synthetic will. It's slower to dry at work after the commute in but it's fine by the end of the work day. And IT DOESN'T STINK!!!

Our morning temps have been in the 40s but it's still not cold enough for fleece. The advice I read on icebike.com (I think) was to feel slightly chilly starting out. That advice is spot on - about two miles into my commute I'm feeling like peeling layers off and this is just a waste of time on a commute, when minutes count.