I don't like copying the initial take and pitchshifting or nudging it back and forth in time, etc. to create the illusion that it's a doubled vocal -- I think a lot of folks here are spot on in acknowledging that it's a very difficult skill to acquire and is the hallmark of a professional -- I certainly don't do it well. Listen to any of Elliott Smith's first three albums: Rob Schnapf (who produced Either/Or) said he sat down, played guitar and sang the song once through, then played the guitar part again while singing the song through a second time. On his records they sound nearly identical but with the right kind of very minute variations, to achieve a very dramatic effect with no special studio wizardry. I know it sound corny but the way to get that good is practice, practice, practice (oh and be born with the gifts of a songwriting god like Elliott was).