When wearing a suit with an open collar (no tie, one or two buttons undone) your best bet is to wear a button-down collar shirt so that the collar "sticks up", holds its form, and effectively frames your face.
The button-down collar:
However, most guys who wear ties on a daily basis generally have a collection of point or spread collar shirts (not button-downs) because these typically are easiest to pair with a tie. The problem is, when you remove your tie the collars on these shirts will flatten out and hide under your lapel, losing your collar's clean neckline.
See below...George's collar is "floppy" and doesn't frame his face, while Brad's stands upright which creates a nice clean line making him look more "put together". See the difference:
Therefore, in the past I would recommend that a man have one collection of shirts to wear with ties (typically with point, classic or spread collars) and a separate collection to wear open under a suit jacket (with button-down collars or large stiff-pressed collars).
The good news is, the solution is here...and it only costs $20. Warning...this might make you think "Damn! Why didn't I think of that!?"
They're called "Wurkin Stiffs". They're magnetic collar stays with small (but strong) magnets that hide inside your collar to effectively hold your collar open and up-right, making you look like Brad below (or at least making your collar look like his).
For more information on how these work, or to purchase them online, check 'em out here.
Yours in style, SB.