Overdubbing can add subtle nuance to a track, or it can turn a single vocalist into a thick, rich harmony. When setting out to record tracks for overdubbing, try to have some specific goals in mind for the session. This goal-oriented approach helps everyone working on the track stay focused and motivated. It can also help to improve the overall quality of your recordings.
You can read more tips about overdubbing sessions here: http://www.prosoundweb.com/article/in_the_studio_the_overdub_checklist/