Add harmony. Harmonizing your song will give it dimension and make it sound fuller. An easy way of experimenting with harmonies is to record your melody line and try singing or playing notes above and below it until you come up with a harmony line you are happy with.
Add more notes in the bass. If your song already has chords or a harmony line, adding notes in the bass will add fullness to the melody. Add a bass drone or experiment with a moving bass line.
Experiment with different instrumentation. Different instruments offer different textures and sounds that can make your song sound fuller. An instrument capable of playing chords such as a guitar or piano can easily add to your melody by creating a rich harmonic accompaniment. Adding more singers to your melody line or incorporating a lower-register instrument such as a cello or bass can also add dimension to your song.
Add effects to your song. If you are recording your song, a basic sound editing program can offer a variety of effects such as reverb, balance and tone boost. Adding effects such as reverb to individual tracks or to the entire recording will enhance your song by adding breadth to all your instruments. On the piano, try using the sustain pedal, which makes notes echo and overlap after you have lifted the keys.