Somewhere when I was studying composition and arrangement I also started to listen critically to music. That, along with furthering my understanding of compositional structures and harmony, is what I would attribute the most important steps towards becoming proficient in composing.
Learning to play a vast amount of music was also immensely impactful as it brought me closer to the work and allowed me to pick up on a lot of tricks.
I suppose neither of those are really tools per se, but it did provide me with the tools needed later in life.