OWB - Oversize good quality 100% Cotton T-shirt untucked.
Good high quality leather holster and good, stiff 1.5" gun belt - behind the hip location - firearm at an all important more extreme forward rake - no vest - no second cover shirt - sure works for me. I like to be Cool & comfortable in hot weather.
It's been my experience that many folks are slightly self-conscious when carrying their firearm (a normal human reaction I guess) and consequently tend to over-conceal and cover their firearm much more than is really necessary.
A good uber high quality leather holster is mandatory and I always go custom these days since most stock "off the shelve" rigs do not "house" a full size firearm with enough forward cant/rake to nicely conceal just under a T-Shirt.