If I really need to nail notes, I will record a guide track of a more pure sound than my own voice. For example, I will record a synth lead melody that I can use to help guide me, otherwise I will sing flat - almost guaranteed.

There are the other tricks you might know about - add some reverb to your voice as you are recording it and feed that into your headphones, pull off one ear of the headphones and do the 'guide your voice from your mouth to your ear with a hand' trick. This helps you hear more than just your bone-conducted head voice, which for many people if they just hear that, will go a little flat (I do) while recording with headphones covering both ears.

For thickening, you can also try doubling either with a sung line timed just exactly like the first, or even whispering the line and turning the level way down of the doubled whispered track, but just enough to support the main track.

-Scott

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