What are your recording levels? They should peak at around 0 dB.
What sound card or audio interface are you using?
What type of music is it?
Is it audio, MIDI or a combination of both? Any software synthesizers?
Adding a little compression can increase the apparent loudness of the music. This can be done on a track-by-track basis or in the Master audio bus (you can't use compression on MIDI tracks, but you can on the MIDI track's soft synth audio outout).
You can also double-track your audio tracks, especially guitars, then pan each track to opposite sides. An example of this is my latest opus,
Leave It Alone with the rhythm guitars panned about 60% each way.
Using too much compression can squash any dynamics in the song, though.