|The Right Way|
|The Wrong Way|
Also, just like underwear, most of your decision-making when it comes to undershirts should be based on functionality. An undershirt and a Tee-shirt are two different things, and should be treated as such. Undershirts, or the good ones at least, are longer, in order to remain tucked, and hug your body in order to refrain from contributing to any of that dreaded billowing of the shirt at the waist. They are also often thinner material, and sometimes are moisture-wicking or have some other sort of textile technology to make you more comfortable. And, like I said, always go for a decently deep V-neck that won't show when you pop the button open in the hot temps.
In a pinch, a Tee will work if you're keeping your button done, but you're just limiting yourself by wearing one. A v-neck Tee instead of a v-neck undershirt is even a bit closer, but still avoid it if possible. It kind of kills the purpose of a well fitted shirt if your bottom layer is all bunched up and billowed on its own. You want your shirt to drape over your body (and your undershirt) like there is no undershirt at all, and that's exactly what good undershirts are made to do.
The good news is there are plenty of great options out there. My personal favorite are V-neck undershirts by Tommy John. I can't remember the exact price, but I believe I got a 2-pack of these from Nordstrom Rack for about $20. I wear them every week, and need to buy more so I don't have to run laundry so often.
They are thin enough to not add too much insulation, hefty enough to absorb my sweat and dirt (lovely, I know). They are also slim fitting enough for a scrawny guy like myself but with enough stretch to not be constrictive. You can buy them direct using the link above, but I got mine at a good discount from Nordstrom Rack, so keep your eyes open.