A few thoughts:
It is important in music to experiment with new ideas. Not only to get ideas, but the ability to expound upon them. Composition deals with details; making sure the score has adequate markings and is readable. It also deals with the larger picture, the form of the piece, and its development from beginning to end.
One number important to all artists is the Golden Ratio. It is used in visual arts but is also used in music composition. It is the standard of beauty in our culture.
I also learned about the different ways composers use phrases. Phrases can be expanded or contracted to fit the desired number of measures. Also, when something is restated, it should be different from the way it was said before.
When dealing with text, a composer makes a choice to emphasize certain words, even syllables, more than others. Repetition and stress can make the meaning stick out to the listener. The use of motives and differences in instrumentation make a difference in how a melodic progression can be memorable.
All these ideas are thoughts I have in mind while revising and improving existing pieces.