I have a wool icebreaker cami w/ built in shelf bra that I wear pretty much year-round. I can't remember if I was wearing it during our hottest days last year, during which I probably would not have been that active.

Probably my favorite time to wear it is at the doctors office -- if I have to roll up my sleeve/take off my shirt to get a shot, or have my BP checked, or to have my lungs listened to, I feel like I'm still wearing something.

I wore a black one of these for most of the week during a recent trip to the Dominican Republic, during which I was hauling around a backpack, hiking, climbing, and splashing around:

http://www.exofficio.com/product_det...1-bed719ee3f2a

It worked pretty well as underwear that could pass as outerwear (I'm rather small), but it was also very comfortable for layering as well. I washed it once during the trip and it was dry by morning, even in that humid climate.