A proper fitting T-shirt

There are 2 styles of t-shirt on the market for men these days: the more traditional t-shirt and the snugger-fitting t-shirt with the goth-inspried graphics.
Regardless of which type t-shirt you choose, it should fit well. The Traditional t-shirt should fit in the chest. If you have a 40 inch chest, your t-shirt should be very close to 40 inches. One of the great things about t-shirts is that they can make your chest look bigger - or at least more proportional to the rest of your body! However, if your t-shirt is too large in the chest, it will make you look even thinner and more sickly. The trendier, more hip t-shirt can be even more snug. Modern looking clothing is designed to be snug so, even if you are thin, the modern T should be snug.
The hems are also different for both t-shirts. The tradional T should fall somewhere near ths spot where your butt starts to bubble out from your body - and maybe a touch below that point. If the traditional T is any shorter than this, it will look silly and make you look too tall. On the modern T, the hemline can be a little higher than the traditional. It's even possible to have some of your belt showing (you wear a belt, don't you?). The snugger fit of the modern T will help it stay in place.
The third thing you need to look at is the sleeve length and fit. On the traditional t-shirt, the sleeve hem should fall somewhere near the middle of your bicep and maybe even a bit below that point - but not to your elbow joint. The sleeve should also fit your bicep well. A good rule of thumb is; if you can fit more than 2 fingers in your sleeve barrell, then it's too big. Nothing will make your arm look too skinny quicker than a sleeve that is too baggy. On the modern T, the sleve can be a bit shorter and even more snug.
I know it's difficult to find t-shirts that meet all these criteria pefectly. So, if you do find a great fitting shirt (and people will tell you), buy one in every color and take care of them.
A few extra principles to follow: It's rarely a good idea to tuck in a t-shirt. T's are casual and can be worn untucked. A dressier t-shirt, with a smoother finish, worn with a jacket could be tucked in.
Hope this helps you look your best!