After wearing crew neck undershirts for quite some time, I decided to make the switch to v-necks.  These Calvin Klein shirts were a bit more expensive but have 5% spandex to make them slightly stretchy and less prone to shrinking in the dryer (like many other cheap undershirts).

They are long enough to tuck in even after a few cycles through the wash and seem to be holding their color quite nicely.  I will definitely be buying some more.