I want to translate this sentence into Spanish:
I could not decide what to wear for Halloween last year.
I was told to use:
No podía decidir qué ponerse para Halloween el año pasado...
but I think this sentence can be contracted down some.
Here in Mexico, this is what I would say:
No sabía qué ponerme para Halloween el año pasado.
I'm from Spain and I would say:
No sabía qué ponerme en Halloween el año pasado
Or, if you are talking about a costume:
No sabía de qué disfrazarme en Halloween el año pasado
It really depends. Because of the ambiguity mentioned in another comment, the sentence could mean something like "I was having trouble deciding what to wear..." leaving open the possibility that the speaker DID decide later. Or it could mean, "I never managed to decide what to wear..."
In the first case, a good bet is something like, "No podía decidir/no sabía qué ponerme (o de qué disfrazarme) para Halloween el año pasado."
In the second case, perhaps something like, "No pude decidir qué ponerme/de qué disfrazarme para Halloween el año pasado." "No supe" doesn't sound right to me, because I've always associated the preterite of saber (supe/supiste/etc.) with FINDING out something, like "enterarse."
As mentioned by others the translation needs the original ambiguity figured out first in order to accurately reflect the intend. Therefore, it depends on the context and the outcome of that situation:
If the decision was never reached (and you wore nothing or someone else decided for you)
No pude decidirme sobre qué usar para Halloween el año pasado
but, if it was only a temporary indecisive stage (and you made your mind about what costume to wear)
No podía decidirme sobre qué ponerme para Halloween el año pasado
There are not contracted ways to state that, which we could think off in Spanish (at least without undermining their expressive quality)