What really matters in your arrangement?
One of the most common issues I see in people's work, especially less experienced music creators, is the creation of just too many parts that they fall in love with.
I love to watch movies with commentary from the director. Sometimes they will show you a deleted scene and say that it was a great scene and that they hated to lose it, but that it hurt the flow of the movie.
The same may be true in your arrangement. You may love that brilliant synth part, but if it draws focus away from a part that matters more, or competes with it, you have to be prepared to be a little ruthless and decide that it has to go.
Know your genre
If you have a song and the most important elements of it are the singer interpreting the lyrics and some lovely piano or acoustic guitar, then you will have one set of criteria. If you are doing an EDM song, your drum machine and synthesizer parts may be the things that your audience will be most interested...