What are the key factors in selecting clothes for use during an exercise program?

Key factors would me depending on the weather and place. Outside on a warm day you would want shorts and tee shirt. Cold day you could want sweats and long sleeves if you wanted. Its really whatever you prefer. Just make sure that when you sweat, it isn't being contained. If you wear a heavy coat on a hot day that could be fatal. 

You always want some thing light weight and breathable like shorts but not thick something breathable. bamboo pulp is a great exercise fabric. i hope this has helped.