In order to keep your electrical panel in order, marking the circuits makes shutting off a circuit breaker a snap. Located inside the door of the electrical circuit breaker panel is a panel schedule. This is a paper that shows individual circuit breaker locations and what is connected to it.
If you open the circuit breaker panel cover, you'll notice the layout of circuit breakers. The left side is the odd numbered circuit breakers that are in a sequence of 1, 3, 5, etc. and the right side is the even numbered circuit breakers in sequence of 2, 4, 6, etc.
Now look on the panel schedule. You'll see that the panel schedule coincides with this layout. Now look at the metal to the left and right of the circuit breakers. Here too are the circuit breaker opening numbers. By looking at the number on the metal, writing the room or appliance connected to that breaker in the panel schedule panel, you'll have an organized circuit breaker panel.
An additional tip to aid in an easier visual is to use sticky labels that can be written on to display the number of the circuit or circuits connected to an appliance or load. Then there is the option of those little number tags. Place these beside the breakers in the panel and you'll have a clear look at the breaker layout.
While we're on the topic of electrical panels, here are a few great articles to guide you through the world of circuit breakers and circuit breaker panels.