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!

The Rules for thin men

Rule #1 - no matter how you are shaped, your clothes must fit. This is the single most important part of looking your best. Your clothes may be older and a bit shabby. They may be out-of-fashion colors. You can pull off a look if your clothes fit. If your clothes do not fit you properly. If they don't flatter your figure, you are doomed to look silly.
I understand that finding clothes is nearly impossible when you're not "normally" shaped, and I'm working on that. You must learn to not buy anything that does not work perfectly for you. I actually had only one dress shirt that I wore for over a year because I could not find another dress shirt that fit me. I saw many shirts that were close, but if they didn't fit just right, so I left them on the rack. Looking your best is important, and buying ill-fitting clothes is a recipe for disaster.