I can only "rock" a sweater in the dead of winter, otherwise I sweat my butt off.
I can't pull off the layered look, cause my panting & complaining give me away.
So IWB strong side with a tshirt un-tucked is my best bet so far.
I am the king of the long baggy sweater in winter... In summer, I'm perfectly happy in a T-shirt and cargo shorts/sandals.
That said, I know SIB was able to hide her M&P Shield without much trouble at all this past weekend. I can hide that same gun under an untucked long T-shirt (Hanes Beefy-T or similar thick cotton) and it disappeared. So, it's easily as much the gun as the holster when it comes to concealing. The Shield has me re-thinking summer carry altogether, since it was so easy to hide at just under 1 inch width in the slide.
What pistol are you intending on carrying, if I may ask?