1 - Generic statements : Imparfait for generic statements using key expressions lacking precision, like " Dans ma jeunesse... ", " À cette époque... ", " À Noël, normalement... "

• Dans ma jeunesse, j'avais les cheveux frisés comme un mouton. Mon professeur de musique adorait ça.
  
  
• Dans les années 90, je n'avais pas le droit de m'asseoir à l'avant du véhicule. À cette époque, je n'étais pas assez grand.
  
  
• À Noël, normalement, nous recevions mes grands-parents à souper. (here, the reader understands that you're talking about the old time, that it's not what you're doing anymore.)

Compare with :

À Noël, nous avons reçu mes grands-parents à souper. (here, the reader understands that you're talking about a specific Christmas, the one from last year).

2 - Duration : In complexe sentences, imparfait will be used with the passé composé. L'imparfait will be associated with duration, while the passé composé will be describing an event happening " inside " the duration.

If you say : Ce matin, j'étais malade. The reader will expect another sentence to complete the idea. Something like : Alors je suis resté à la maison. " Je suis resté à la maison " is a specific (precise) decision that was taken at a specific moment while the person was feeling sick (duration).

But if you say : Ce matin, j'ai été malade. Then the reader is not expecting more info. It's a closed statement, not considered as a duration, but as a specific event.

• Où étais-tu ce matin? (duration) - J'étais à la recherche de mon chien. (duration) Je suis revenu trop tard (precise moment) et j'ai manqué mon autobus (precise moment).
  
  
• Les années 90 ont donné naissance à de très bon groupes (" ont donné naissance " is an activity that gave results at specific moments inside the 90's). La musique était excellente (general statement, vague, valid throughout the duration).
  
  
• Marc et moi, nous discutions depuis une heure, quand le téléphone a sonné. C'était sa mère (duration). Elle le cherchait (duration). On a compris (at that specific moment) qu'elle était fâchée (duration - unfortunately!)

BE CAREFUL : a duration here does not mean " any period of time ". No, it means a period of time in which something else is happening or could be happening. Therefore you'll see the passé composé used to say Nous avons discuté pendant une heure, even if you might be tempted to consider " pendant une heure " as a duration. It is, of course, in a way, but not according to our definition for the imparfait. Here it means that the discussion is considered as an event, not a duration, and moreover a " finished " event.

3 - Sometimes, you have the choice :

• J'ai aimé les années 90. (passé composé, because these years are over, completely finished, you're showing a real cut between then and now, which doesn't mean you don't like the 90's anymore.)
  
  
• J'aimais les années 90. (imparfait, because here you choose to present your appreciation as a duration, and the 90's themselves appear more like a larger period of time).
  
  
• In both cases you could continue by saying : Et je les aime encore. / or : Mais je ne les aime plus.

Most people would say that the meaning is the same.

Another one :

• Hier, je recevais mon premier bulletin de l'école secondaire (you choose to give the angle of the duration). Mes notes sont bonnes.
  
  
• Hier, j'ai reçu mon premier bulletin de l'école secondaire (you choose to give the angle of the specific, completed, event). Mes notes sont bonnes.

Here again, most people would say that the meaning is the same. I would argue that by using the imparfait, you put more emphasis on the event, by insisting on its duration, by " spreading " it. In the second one, you seem to care less... so I guess you would use the second one! (just joking)