I’ve spent the past 20 years singing jingles for radio and TV adverts. Part of the trick to getting that rich full sound is to stack the same vocal line over and over. It’s often called ‘double tracking’ , and although you need to match the previous recording pretty darn close – those tiny differences in performance create that thick creamy sound.
A jingle can often be sung by just one singer, but in the ‘vocal stack’ there might be three or more lead vocals, and then each harmony line would be at least double tracked – so pretty soon you can have a stack of easily 8 or 9 tracks – sometimes many more.
For me the best double tracker ever was Karen Carpenter. So flawlessly done you can hardly tell the lead vocal is ‘multi-tracked.’
As for recording technique when you’re stacking vocals, it can really help to close one ear off with your finger to make it easier to hear yourself whilst at the same time hearing the previous take on the other side of the headphones. Also stepping gradually further from the mic as you double track a vocal line, can help create a sense of space and texture. See the video of Michael Jackson working with Quincy Jones for an inspiring demonstration.