What happened? (Debug/Verbose, Info, Warning, Error, Critical)
When it happened. (Dates but don’t forget timezone offsets)
Why did it happen? (who did what and where)
What you should do next. (is there a KB? Does this happen often)
Those are the four components of any great log file, if you have those in your file the people that have to go troubleshoot will thank you kindly.
The fifth component of any great log file is ensuring people know where to go and find the information, disseminate it, and do something with it.