I don't switch from shorts to pants until it's cooler than 30 degrees. You lose relatively little of your body heat through your legs.

On colder days dress so you're chilly the first 2 miles. Otherwise you'll sweat to death before you arrive.

Even in sub zero temps I don't wear anything heavier than a wind breaker over my long sleeved t-shirt, and that shirt is soaked with sweat when I get to work.