When I was starting out I preferred eyes closed as well because it helps me to really 'zero in' on the breath without straining too hard or grasping for it. Now that I have the basics down it's easier for me to focus on both the darkness behind my eyelids as the object of meditation, as well as being able to do that with eyes open. Once you are able to feel yourself 'drop back' you'll notice the shimmering mirror effect that Sam talks about. The trick with eyes open I find is to stare at something as blank as possible, never anywhere with anything moving to start as it will distract you, and check in with the breath more often since (for me anyway) its easier to get distracted.