sorry for the short reply - yes, layers, but they're more obvious for the winter. layers are important in the summer too - with a good base layer you can stay cooler than if you were just wearing a jersey.

lots of people have recommended Craft, i have one cool-weather base layer from them that's awesome, but they're a bit expensive. others have pointed out that you can get good non-cotton layers at walmart/kmart/target/whatnot.