Labeling Your Electrical Panel
To thoroughly map or label every circuit in your home will take up more space than is available on your home electrical panel itself. On the other hand, you don't want to keep the record you will make away from the panel (in a notebook or in a computer), because the next homeowner (or anyone servicing your system) may be deprived of your hard-found data.
So I recommend creating a chart of some kind that will mount to the panel, is proportioned like the layout of the circuit breakers (or fuses) themselves and leaves you plenty of room for detailed descriptions. Or it could be an actual map of all your rooms, with circuit numbers placed where the electrical items are. I also recommend using pencil rather than ink, so that changes, additions, or rearrangements are made easy over the life of the house.
By now electrical Code requires good labeling of any new breaker box or circuit: "Every circuit and circuit modification shall be legibly identified as to its clear, evident, and specific purpose or use. The identification shall include sufficient detail to allow each circuit to be distinguished from all others. Spare positions that contain unused [breakers] shall be described accordingly. The identification shall be included in a circuit directory that is located on the face or inside of the panel door... No circuit shall be described in a manner that depends on transient conditions of occupancy." (2011 NEC 408.4A)
There are two common ways of numbering the circuits, but if no system is already apparent (e.g., stamped into the metal of the cover), you are free to make up your own.
Now, the best procedure for an accurate inventory of your circuits is a systematic one. Whatever way you go about this task, I want to forewarn you that your work will be incomplete and you will need to repeat steps unless you first determine two things about your entire house.
First you need -- for your own sanity -- to list each and every electric item in or around your home. You are not going to find that each circuit corresponds exactly with a nameable room. So you need to think in terms of each receptacle, each light, and each built-in appliance. And don't forget things like wired smoke alarms, a security system, the doorbell, a furnace, electric heaters and water heaters, pumps, lights under the house or in an attic, outdoor and garden lights and receptacles, and things controlled automatically (by thermostats, photocells, motion sensors, and timers).
Second, you will need ways to determine whether each of the things listed is receiving power or not. Using a portable lamp or nightlight makes sense for receptacles, and switching most permanent lights on or leaving their switches on makes sense (if your light bulbs are all functional). But other items may take more doing; think ahead about this.
A common reasonable procedure for determining which things a given circuit consists of is to turn off a single breaker and then go check for what is dead. This will work if you are sure to check everything in the house for deadness. Another way is to turn everything in the panel off and then turn on just the circuit to be described. This method forces you to see signs of life in a thing before listing it, and you still need to check everything in the house. In both cases, after you have listed everything in the first circuit or two, the number of remaining things to keep checking gets smaller. Decide which one method you will use and stick to it.
You might encounter some uncommon results. If there are items which will not turn off for any one breaker (or which turn on for more than one), some kind of miswiring has occurred and needs correcting. Another thing to look for would be if one of the large "double" breakers turns off (or on) quite a few lights and receptacles (one-third of the house or more). This may indicate either: 1.) that this breaker is a "submain" (and needs that label) and simply sends its power to most of the single breakers in the panel; or 2.) that it is feeding a "subpanel" (which has a number of circuits of its own) elsewhere in the house, which you should locate and should label as well; in this case, the double breaker at the main panel should only be labeled "Subpanel", also telling its location.
In indexing, labeling, or mapping your electrical circuits, try to avoid descriptions like "Aaron's room" or "TV room", as these designations can change and will be no help to another owner. Try using terms like "receptacles on north wall of west bedroom".
Happy hunting! ☺