Insert a software or hardware equalizer onto the vocal track. The process for doing so will depend on your recording or mixing setup.
Decrease the gain of the equalizer around 200 Hz to remove muddiness from a vocal. Doing so will add clarity to the vocals.
Increase the gain of the equalizer around 3 KHz for added clarity to the vocals. Be careful not to increase the gain too much in this area, as this can cause the vocals to sound "hard" and unpleasant.
Reduce the gain of the equalizer around 3 KHz to enhance the "breathy" sound of the vocals.
Increase the gain of the equalizer around 5 KHz to add presence to the vocals.
Increase the gain of the equalizer above 10 or 15 KHz to brighten the vocals and enhance the "breathy" or "airy" sound of the vocals.