I really like hydrophobic, skin-tight mesh undershirts. I have a shirts from Craft and Giordana I wear most often, the cheap ones from Amazon are fine too. I'm a particularly heavy sweater and prone to salt-buildup and chafing. On long rides this can become such an issue that I almost need to stop. I find I can can wear mesh undershirts even on the hottest days and they prevent any chafing of my upper body. I don't find they make me much warmer.