Every time I start working with my students on the uses of these two verbs I hear the same thing from those who had studied the topic in the past:

“We use ser to talk about things that are definitive, that cannot change and estar to talk about those things that can change”

Unfortunately this is incomplete and not true on many cases:

Mi padre está muerto (will this situation change?).

Y mi hermano es estudiante (and he will not be one forever).

The uses of these two verbs are much more complex than that. The first thing you should remember is that this discussion is only valid when talking about ser and estar with adjectives. For instance, when talking about nouns, only the verb ser is used. The verb estar cannot be used with a noun.

María es empleada en un banco.

Lo que yo necesito es un vestido que sea rojo y largo.

With adjectives, we will use ser to talk about what defines the person or thing. Those essential characteristics are normally expressed with ser. This verb tells what my nature of being is. It works like an equal sign.

With adjectives, the verb estar expresses a condition, a situation, regardless of the duration of the action. That is why the example above (Mi padre está muerto) is correct. We use estar to mention the state of the subject.

A tip I usually give my students when they are hesitating about which verb to use with an adjective is to try first the verb encontrarse. It means “Hallarse en cierto estado”(e.g. Encontrarse enfermo). In English, to find oneself on a certain state. With adjectives it has a meaning very close to that of estar. As a result, if the sentence makes sense when using the verb encontrarse, that means we will need the verb estar. If the statement looses all of its meaning when using that verb, then the verb will be ser.

Let´s see two examples: if you are thinking of a way of saying that you are happy at your job, you can say:

Yo me encuentro muy contento en mi trabajo.

That sentence makes perfect sense. Therefore, we can also say: Yo estoy muy contento en mi trabajo.

Now, let´s try that verb again to express that your trip was perfect.

El viaje se encontró perfecto.

This doesn´t make any sense (or at least, we can say that the sentence is not expressing the idea we have in mind). Then the verb is not estar, but ser (El viaje fue perfecto).

With practice you will get used to this technique that works well when deciding which of these verbs to use with an adjective.

But remember, this is just one of the aspects of these two verbs. The picture is much larger.