Quote The Elf:
Record one vocal and tune it to death (I use Cubase's VariAudio, but Melodyne will do). Now sing to that horribly tuned vocal as a guide.
It's not always going to work, but with some vocalists I work with this has improved their performances beyond all recognition.
I do this a lot. Especially with harmonies. Sometimes I use it as a guide vocal, but lately I find just singing along to it in the car prepares me for a (half) decent take. I also find taking a break between tracking & appraising what I've done helps to keep me objective.
Guns don't kill people. Bullets do.