There are three factors that can draw the eye to a particular area. They are:
Here is the original photograph. The hand is very distracting.
I selected the butterfly, feathered the edge, then selected the inverse. (Select - Inverse)
I blurred, darkened and desaturated the area outside the butterfly.
Your eye is drawn right into the butterfly. I overdid this a bit just to make it easier to see. You'll want to create a more subtle effect by feathering your selection out more.
The image below shows these intersections which are cut into thirds from left to right and from top to bottom. The resulting intersecting lines are where you want to place your subject as shown below.
Fortunate for us, Photoshop automatically shows these intersecting lines when cropping. This makes finding these focal points much easier!
For more information on the rule of thirds see my previous post on the Rule of Thirds.