Problem with mirrors : once you don't have them anymore, you can't see what you look like, and thus don't know what to correct to look your best.
If you can, practice in front of a camera, have somone take your picture and see what it looks like, try to correct it, and so on. In that way, you will learn how the pose feels in your whole body.
Also, having someone take pictures of you for practice will get you used to having your photo taken. Almost all cosplayers I know started awkward, and gradually got used to cameras pointed at them. In order to feel relaxed and give your best in front of a camera, you need both self-confidence, and trust in the photographer. Both build up as you get more practice.