Read from the bottom
The newest entry is last. Start at the moment the problem was reported and work backwards, rather than from the top of a file covering three weeks.
Separate notices from failures
Deprecation notices and warnings are noise on most sites. Fatal errors, uncaught exceptions and 500 responses are what actually broke something.
Match the timestamp to the report
The single most useful step is correlating the log entry with when the customer said it happened. Without that you are reading someone else's problem from last Tuesday.
