Get an athletic-cut shirt.The thing with me is, while do have a fairly muscular build, I'm not really lean. And because of that slim fit is a really awkward fit for me and I usually have to go up a size, then you have the problem of the back of the shirt flaring up like a fukking dress.
Getting a slim fit (regular chest with narrow waist) shirt is not as tapered as an athletic-cut (wider chest and narrow waist).