Draw a tick-tack-toe board within this picture you will see where the lines will go & intercept.
For this one, do the same.
This neat little trick also works on landscapes. However with landscapes make sure the horizontal line is on the horizon of the land scape.
Over all, it's something to do while taking any kind of pictures. Coming from a person who doesn't take pictures often, I find it to work wonders! Try it yourself and don't be shy to share with me!