If you need to pitch correct a vocal but don't want it to sound like the vocal was pitch-corrected, you should use a few settings to smooth out your autotune. The first thing you should do is raise the return speed. I often use a return speed of 30-220ms if I want a vocal to sound natural. Next, enable natural vibrato. This will allow notes to waver in pitch rather than holding them to the exact right frequency. Finally, some autotune software has a "humanize" setting that controls how and where the effect is applied.