Luckily we don't get very cold down here. When I first started commuting I bought expedition weight polypro top and bottom, but they are too warm at 30F which is about as cold as we usually get. Over the past couple of years I have purchased a number of lightweight Duofold Coolmax t-shirts long and short sleeves on clearance from sierratradingpost.com. I just put on as many as I think I need and it works great. This morning I had my usual short sleeve undershirt plus 3 with long sleeves. It might have been one layer too many, but I sure didn't sweat much.