I'm using this technique with a button down shirt in this tutorial, but you can also use it on plain pullover shirts too.
Measure how much you need to trim and where. I needed about 2 inches at about rib height. Mark on the inside of your shirt. Starting at the bottom corner, free hand a slightly curved line, tapering in, then curve back out to the armpit as in the picture. You can see that I took a little bit of width off the sleeve too.
Sew up your line then cut off extra fabric. See the difference?! Repeat on other side and you're done!