Think about what you do when you take a photo. Do you press one eye to the viewfinder and shut the other eye? While that seems to be the natural thing for most of us to do, it turns out that keeping the other eye open may yield some interesting benefits.
I think it's natural to close one's other eye when a camera is brought to one's face; after all, it's a hulking chunk of metal, and we're generally hardwired to close our eyes when things get uncomfortably close to them. Plus, we feel like we're somehow focusing on what's in the viewfinder more by doing so. Nonetheless, as this helpful video from David Bergman shows, there's something to be said for keeping one's non-viewfinder eye open. I started doing this a few years ago and have found it has vastly improved my ability to anticipate action outside the frame, increasing the amount of time I have to react and making it easier to get the shot I want. As an added bonus, I've found that it's easier to maintain my balance when rotating my body to pan with a subject or some similar motion. Give it a try!