How do i decide whether to use ser or estar?

In general, ser is used to describe permanent or innate qualities where as estar is used to describe something more temporary. So geographical location, weather and moods and continuous actions are described with 'estar', whereas things such as someone's job or physical description is generally described using 'ser'.
An exception to this is when you're telling the time: to say 'it's 9 o'clock,' you would say 'son las nueve', using ser instead of estar. I always find this strange as time is the one thing that's constantly changing!