Finding circuits on breakers
"Breaker Finder" (mentioned above) or "Circuit Breaker Detector" is a must-have tool as well as a label maker IMO. I bought an inexpensive detector from Harbor Freight for under $20 thinking that it was a one-time use. Two houses and countless labels later, it's still there when I need it. Also, you might want to grab a lightbulb socket to outlet adapter for ~$1.00 and use the detector's receiver in that.

Another suggestion is to see if there are multiple wires into the back of a breaker. It's an illegal practice and potential safety hazard (although some people may debate this.) It can screw up readings from breaker detectors as well as leave you confused why other areas of you home might are affected.

Also, panels are not allowed to have completely empty slots (holes.) instead of a metal slot cover, someone might've stuck an unused breaker in there to fill in a void.

Last but not least, someone might've left a breaker in but terminated/capped off the wiring. If so, hopefully it's at least in an accessible junction box.