This for days.
I have pretty big shoulders to begin with. The suckiest part is that measurements-wise, I should be wearing a medium. But even when I can get a medium on, it doesn't sit right because of my shoulders. If you have the cash, I'd say go for one that's a size up from what you currently have. Wear that one regularly while you work on breaking in the smaller one. It's good to have more than one anyway.
Try putting it on both ways (from the top down, or from the bottom up). When it's at the point on your body where it's at the most stress (probably around your shoulders or around your hips), pull on it a bit. You don't want to stretch it too much, but a little loosening should help. Keep in mind that it might feel smaller after coming out of the laundry.