I am kind of agree Chisel in this regard of creating frictionless surfaces in the heat, though I never wear cotton underwear. My normal wear options are tighter boxer briefs by Ex Officio or recently Under Armour. I run and do most outdoor activities in triathlon bottoms.

I use petroleum based lip balm, like Vaseline or generic and carry it on my bike, in my running belts and EDC packs. It is small in size, and in dire need will help with chaffed lips, nipples, groin or inside of arm issues due to heat rash.

As an endurance athlete in the desert Southwest I have run into my issues with body glide melting in the car, thus I use Two Toms SportsShieldlove this stuff for preparing larger chaff potential locations. Its like teflon for the skin.