one thing i read about recording vocalists is that its good to place the mic upside down and so that the vocalist's mouth is firing slightly upwards...this encourages the singer to keep their windpipe straight and open...
dont quote me on this one (maybe someone can correct me if im wrong) but if you want to bring out the treble in the voice, place the mic higher, and if you want to accentuate frequencies lower in the spectrum, move the mic lower.
(by the way i dont claim to be an expert but i am recording an rnb guy right now)...but i usually apply the same logic as recording rappers...stacking on parts where you want to emphasize. also learn your harmonies...learn how to show them to your singer on the keyboard...they might only harmonize in octaves and harmonizing with sevenths and fifths might just do the trick for a certain part...
hope this helps