There are two questions here: how to use "se" and how to use personal pronouns. "Se" can sometimes be a personal pronoun (as in "la mujer se mira en el espejo") or just a pronoun with no "personal" meaning (as in "no se deben sacar conclusiones precipitadas"). You know "se" is working as a personal pronoun when you can change the person and the pronoun "se" changes as well. Consider:
La mujer se mira en el espejo ➡ Yo me miro en el espejo.
- "Se" changes to "me". Therefore, "se" is a personal pronoun here.
No se deben sacar conclusiones precipitadas ➡ No me debo sacar conclusiones precipitadas (?)
- Changing "se" to "me" renders the whole sentence nonsensical. Therefore, "se" isn't a personal pronoun here.
"Él se había ganado un premio" is a case of personal-pronoun "se" (see how "se" changes to "me" in "yo me había ganado un premio"). Therefore, your question isn't about the pronoun "se", but just about personal pronouns.
That said, knowing when to use personal pronouns is one of the trickiest things about Spanish. I identify four cases:
- Verbs that never take a personal pronoun.
- Verbs that always take a personal pronoun.
- Verbs that sometimes take a personal pronoun, but the meaning remains essentially the same.
- Verbs that sometimes take a personal pronoun, but the meaning changes.
Unfortunately, there's little or no way to know how a verb works or in which category it falls. Transitiveness/intransitiveness is only a part of it, to which I'll come back later. Also, not even these categories are clear-cut. Consider the verb "ir/irse". Both "voy al parque" and "me voy al parque" translate to "I'm going to the park". I'd consider it a Case 3 verb. However, "voy" and "me voy" are clearly different, "voy" meaning "I'm coming" and "me voy" being "I'm leaving". In that case, I'd consider it a Case 4 verb. So adding a circumstantial object changes the classification of the verb, which doesn't conform to the maxim of "circumstantial objects are merely circumstantial; omitting them doesn't change the essence of the sentence".
[If I get to find one, I'll post a list of some common verbs and the category they generally fall into.]
Antes él (se) había recogido las flores
In "Antes él se había recogido las flores", you are dealing with a personal pronoun as well (consider "yo me había recogido las flores"). "Recoger/recogerse" would fall in the Case 3 category. In this case, the personal pronoun is used when the object that is being gathered belongs to oneself, but it doesn't change its meaning. Other examples of such usage are "me duele la pierna" (my leg hurts) or "no me veo las manos" (I can't see my hands). So this sentence would actually translate as "He had gathered his flowers earlier". Note this use is pretty much restricted to parts of one's body or things intimately related to it, such as one's clothes. If the flowers belong to him but are in the ground, then "Antes él se había recogido las flores" will sound odd. If the flowers, however, were a part of his costume, then it would sound natural.
Él (se) había ganado un premio
In "Él se había ganado un premio", I would consider "ganar/ganarse" to be somewhere between Case 3 and Case 4. Here, the personal pronoun would be an ethic dative — not a reflexive pronoun. Whether the meaning changes or not is debatable. I personally feel "ganar" and "ganarse" are different things: "ganarse" is more related to one's behaviour, and may come across as a slightly patronizing thing to say depending on the context. "Ganar" would be "win/earn", and "ganarse" would rather be "get oneself". Therefore, they aren't always freely interchangeable:
- Phelps ganó varias medallas de oro. ("Phelps se ganó varias medallas de oro" doesn't sound natural. Here you can see the "win" part of "ganar".)
- Pepe se ganó una mala reputación. (Again, "Pepe ganó una mala reputación" wouldn't sound too natural. Here you can see the "get oneself" part of "ganarse".)
Transitiveness vs intransitiveness
Regarding transitiveness/intransitiveness, when a verb generally takes a DO, but in a certain sentence it doesn't, then a personal pronoun is added.
I move the vase = Muevo el jarrón. ➡ Move/mover is transitive.
I move = Me muevo. ➡ Move/moverse is intransitive.
I lean the tower = Inclino la torre. ➡ Lean/inclinar is transitive.
The tower leans = La torre se inclina ➡ Lean/inclinarse is intransitive.
If the verb can't take (or rarely takes) a DO, then generally you don't need a personal pronoun.
I run = Corro ➡ Run/correr is always intransitive.
I jump = Salto ➡ Jump/saltar is always intransitive.
Of course there are exceptions. Just to name one, "caer" is always intransitive although it generally takes a personal pronoun:
I fell (off) my chair = Me caí de la silla.
In conclusion, to know whether a verb takes a personal pronoun or not and what difference it makes you first have to know to which category it belongs. A lot of personal pronouns are actually just ethic datives, and can be omitted without the sentence changing its meaning.
In English, ethic datives are rare compared to Spanish, and they don't work the same way. A canonical example of Spanish ethic dative is "comerse" in "me comí una hamburguesa". If you consider that "me" a Direct or Indirect Object you would be saying you ate yourself. Conversely, ethic datives have no syntactic function in the sentence, let alone a DO or IO. Therefore, they can be freely omitted with little or no change in meaning, but of course not all verbs can take an ethic dative. When they can and when they can't is, again, impossible to know.