Mix the track down in Acid, which involves using the "Export" command under the "File" menu. Clean the track up before exporting and add all individual track effects as well as compression before starting the mastering process. Mixing the track will take all of the individual tracks and create a single stereo file for mastering. Open the new track in Acid for the mastering process by clicking "File," "Open" and locating the new stereo track.
Add overall equalization to the stereo track. Choose a graphic equalizer from the "Effects" menu, which is located under the "Tools" menu, to fine-tune the EQ, or use a para-graphic EQ on the mixed vocal track. Use minimal EQ so as to not take away from the fullness of the just-mixed track.
Use a multi-band compressor, also found in the Effects menu, to tighten up the different frequency ranges of the track. Mastering adds a nice, clean sound to a mixed track, so using a multi-band compressor will allow you to tighten up the bass frequencies. Open up the treble frequencies by lowering the compression in that frequency range for a nice, crisp sound, and accentuate the midrange frequencies for radio use by lowering the threshold.
Use a stereo imaging program to make the track seem fuller and more open. There are plug-ins from various companies that can be used in Acid to make your vocal track seem wider and more open and accentuate the stereo field, but use them judiciously, as they can cause your vocal track to sound muddled.
Consider the use of a mastering plugin such as iZotope or Voxengo, both of which have all of the above included and can normalize the track as well, which raises the volume of the track. iZotope and Voxengho have presets used by some of the best mastering engineers out there. Starting with a preset can get you most of the way to a quality mastering job with one click. The mastering plug-in will be found in the same menu as the effects used above.