I have gone down many blind alleys by listening to what HO says
but
I have occasionally missed something obvious by not listening
so listen but don't believe
Good advice, as not all the time, but this can lead to important clues, also if they are on there own circuit make sure you pull the breaker out and look at the buss stabs and buss, especially with aluminum buss ITE's and T&B panels, back stabbed receptacles, or a bad connection in one of the lights that heats up when a certain light is on for a while (heat from the light) if they are working when you get there, just look for the common stuff as many have posted, loading the circuit is also a good method as was said but becareful and keep a keen nose out for any burning smell as you never know what splices might be hidden behind the walls, try not to wast allot of time, or it could be a good future customer killer, best to tell the customer to call you when they quit working again than rack up allot of time trying to find a needle in a hay stack, of course look at the common things, and a good wire tracer goes a long way to finding everything on a circuit, and is a must when the circuit stops working.
Nothing like trying to fix a circuit thats not broke when you get there!
Kind of like looking for a noise in your car when it wont make it when your looking for it.