One tip that instructors often pass onto the beginning photographers is to use their dominant eye (i.e. the eye they prefer seeing with) to look through the viewfinder.
If you want to find out which of your eyes is the dominant one, here’s a quick test you can do: extend your arms straight out and form a small triangle with your hands. Looking through the triangle with both eyes open, frame something nearby (e.g. a doorknob) and place it in the center of the triangle. Then close your eyes one at a time without moving the triangle — your dominant eye is the one that placed the object in the center. Interestingly enough, many people (myself included) choose to use their right eye for their viewfinder even though the left one is dominant — likely because it’s the way they started shooting from the beginning. (via Reddit)