The imperfect is used for actions in the past that are not seen as completed. Using the imperfect implies that the action did not have (or we don't know about it) a definite beginning/end or both, or that the beginning or end of the event is not important or relevant. Many times the imperfect is used to express a repetitive or habitual action that occurred in the past.

1. For actions that were habitual/customary
(used to/would)
Escuchaba la radio todos los días.

2. For actions that "set the stage" for another past action. (description of background information)
Siempre iba a la playa

3. An action in progress 
(mientras/durante)

Él trabajaba en Costa Rica...
Ella estaba en su oficina...
Hablaba con mis compañeros...
 

4. For telling time
Eran las dos cuando...

5. For stating one's age
Cuando tenía 16 anos....

6. Usually for mental states (creer/querer/poder/estar)
Quería ir al cine ayer.

7. To describe people, things or conditions
Mi perro era activo y simpatico.

Clue words
A menudo=often
A veces=sometimes
Todos los dias=everyday
Cada ano=every year
Generalmente=generally
Muchas veces=often
Siempre=always
Todo el tiempo=all the time 

The preterite is used for actions in the past that are seen as completed. Using the preterite implies that the past action did have a definite beginning/end or both (or that we are clear about).
 
 
 
 
 
 
 
 

1. Actions that were repeated a specific number of times.
Visté a mi amigo Juan tres veces el mes pasado.

2. For actions that occured during a specific period of time.
Fui a la playa tres veces
 
 

3. Interruption of an action in progress-action that caused the interruption

cuando su hijo nació.
cuando su esposo llamó.
cuando oí las noticias.

4. Chain of events, one after another.
Esta mañana me levanté temprano, me bané, desayuné y salí  para la escuela.
 

5. To state the beginning or end of an action
Empecé a estudiar a las 3.
 
 
 
 
 

Clue words
Ayer=yesterday
Anteayer=the day before yesterday
Anoche=last night
La semana pasada=last week
El año pasado=last year
De repente=suddenly 
En seguida=at once
El otro dia=the other day
Por... anos=For... years