On my longer rides (non-commuting) I wear a very thin silk-like undershirt that is sleeveless (cant remember the brand). It picks up a bit of sweat and acts as a cooler. I use the same thing in winter to keep me warm. They are awesome.

Second, I slow the pace a tad. Just enough to keep aitvfolw, but not so hard as to overheat from the inside out.

Last, always light colored clothing. No black bike shorts. Usually REI nylon shorts that are very light weight too.