Record the vocal track. Freeware recording software like "Audacity" can do many things well. Click "Record" in your recording software and record the vocal on a separate mic. Devote several audio tracks for the vocal in your audio sequencer.
Double the vocal track. Double tracking is a technique to both strengthen a vocal and improve its intonation. Play the recorded track in the headphone mix and have the singer sing along with it. Depending on the effect desire, you can do this a third time as well.
Center the vocal track. When mixing, center placement actually places the centered audio track to both the left and right. This can serve to draw attention to the vocal, especially if the instruments are positioned to the sides of the mix.
Add vocal effects. Echo and EQ all "sweeten" a vocal track to increase its audibility over music tracks. Pitch correction processing subtly bends the note to the exact perfect center of the pitch.
Add parametric EQ to the final mix. Parametric EQ adjusts both the frequency and the bandwidth. It permits a touch up to the frequency response of the vocal's presence -- and anything else within the adjustment band. Subtle enhancements can have a dramatic effect.