Look for crewnecks that fit well throughout the shoulders; make sure the armholes don't sag or pull throughout your armpit and that the shoulder seams are smooth. Because a crewneck covers more surface area than other necklines, avoid builky sweaters. Go for thinner, slim-fitting versions for maximum, elongating flattery. Finally, make sure the sweater stops at your hips - any longer and you'll look swathed in fabric.
Simple details can add some ooomph to the basic crewneck shape, like the Henley buttons on this sweater from Lane Bryant. Added bonus - you can re-style the sweater by leaving a couple of the buttons open and layering it over a pretty tank.
Photos courtesy of Target, Talbots and Lane Bryant.