Uses of "ser": Ser is used to classify and identify permanent or lasting attributes. If the general rule isn´t specific enough for you, think of the acronym DOCTOR, which stands for Description, Occupation, Characteristic, Time, Origin, and Relationship.
Uses of "estar". Estar is used to indicate temporary states and locations. If the general rule doesn´t suffice, think of the acronym PLACE, which stands for Position, Location, Action, Condition, and Emotion.