The Winter Cycling forum has tons of advice on what to wear in the cold. It's very easy to stay comfortable in cold weather, as long as you are moving on the bike. It's not so easy to stay cool on the bike in the Summer.
As mentioned, cycling generates heat. It will never generate cool air, at least not enough to cool you on a very hot day. Your best bet in the summer is to still wear a light wicking jersey and cycling shorts and to stay hydrated. Another great tip is to invest in a Halo headband because sweat in the eyes is one of the most uncomfortable parts of Summer riding. The Halo pretty much eliminates that.