keep arrangement simple but interesting using different elements musically.
if you listen to orchestral/classical music, this will be very apparent. it'll apply to contemporary stuff like Pop, Rock, Rap, RnB, etc.
having a knowledge of what kind of sound is created with what inst helps a lot.
an example with a certain section of song. percussion has character with lots of transients, strings can be more drawn out (ie, pads), woodwinds can be doing melody, while brass can be doing a loop-like figure.
this way, each section of orch is doing different stuff; and if written well (meaning good relationship between all elements doing musical tension & release), they won't sound cluttered when played together.
of course, those same inst can swap characteristics. meaning strings can do the melody, brass do pads, etc.
the inst can have different combinations as well, so flute playing same line as violins, while brass is doing similar hits with triangle. options are almost endless.
apply the same idea with any music.
imagine you are drawing on a piece of paper: draw a few straight lines across the page (pads), put some dots under those lines (percussive transients), scribble a few zig-zags (loops) up top, and finally a flowy line or a big fat square in the middle.
use different colors for each element.
this way, you can tell each apart. you can layer them, or you can keep'em all separated. nobody said they all gotta be there at the same time; take one or two elements out.
that's how i "see" music. it's not that much different for all genres, to a certain degree. if you draw enough times, you'll start to see what makes a good picture, and what doesn't.
so i would think a good arrangement is an organized and clean picture. not to say, this equals a good song tho. most of the time, but depends.
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