Since I became an adult I stopped using an undershirt in the summer (against mom's advice).
Lately I've read this:
"The Role of an Undershirt
For both the short sleeve and long sleeve button up shirts, always wear a 100% cotton undershirt (V neck preferred). Although wearing two layers of clothing sounds warmer than wearing a single layer, a cotton undershirt protects the outer layer from sweat and fabric staining antiperspirants. If you sweat heavily, consider bringing an extra shirt to change into. "
from The Art of Manliness: http://artofmanliness.com/2009/07/23...-dress-part-1/
So, I'll give it a try this summer... and I'm thinking that mom could be right first time... like in many other things