Most shirts are "classic fit" or something along those lines. Well fitting shirts for slim people are "fitted" or "slim fit" or some such. "modern" is usually in between. The same goes for tshirts and pants. http://www.primermagazine.com/2011/field-manual/how-to-wear-a-tucked-in-shirt-without-looking-like-an-old-man

You can also get a tailor to slim your shirts. This runs about 30 USD/shirt at a tailor shop but sometimes you can find people offering such services on craigslist for less.