With so many virtual instruments available today its very tempting to start adding endless layers of strings, brass and synths to your demo before the foundation of the song is finished.
In order to make the process more efficient make sure to ask yourself the following questions before you start working on the arrangement :
What is the song about?
What are you trying to express with this song?
What is the basic melody?
What are the basic chords that accompany that melody?
Which octave are either of these on?
What is the beat and tempo of the song? Which key is the song in?
Are you comfortable singing in that key/ tempo?