Listen to the vocals without any tweaks, but mix them at a comfortable level just above the rest of the music. Allow the vocals to sit on top of the mix a bit louder than you would normally mix them to hear if you need to make any adjustments or tweaks to them before you mix the entire song. Play the song back at a moderate level and listen carefully for any low parts or areas where the vocals sound unnatural.
Compress the vocals slightly to make sure that all of the syllables are heard but make sure that the soft parts remain dynamic. A 2:1 ratio is a good start, making sure you have a fast attack and a medium release time. The attack dictates how quickly the compressor kicks in and the release is how long it remains after the signal has dipped below the set threshold.
Make subtle EQ changes to improve and tweak the vocals using a graphic equalizer. There are many ways to do this, but make sure you retain the natural quality of vocals. A boost of 3db at 3k Hz is standard for vocals while a dip of 3db at 1k Hz can minimize the nasal quality of some vocals when that sound is not desired. Use a high-pass filter to eliminate all frequencies below 100Hz to clean up the bottom end.
Add reverb to the vocal channel. Reverb adds depth to the source material, so it works well on vocal tracks. Add just enough so that there is a richness to the voice, but not so much as it sounds as though the vocals are in a different room as the music. Rock and pop vocals always have some reverb, but not as much as other genres where expansive reverb is common. Plate reverb is the most common type used on vocals regardless of the style.
Mix the song down with the vocals at an even level with the music making sure you can hear all of the singing properly. Vocals are often considered the lead part of a song, so they must be clear and sound natural in conjunction with the other music. The vocals should be clear, but not at the expense of any other element of the music, so be aware of all of the other instruments and key sounds, including the snare and bass drum as well as the bass guitar.