A more simplistic, and still very effective approach, is to just use a nice stereo delay (with a different delay time in each channel) for a slight thickening or doubling. Many delay plugins also have modulation which can be added to further thicken the sound.

I find that unless the singer is really good at doubling their voice, double tracking the vocals is just a waste of time as it rarely sounds as good as one track with a stellar performance.