Sustained Notes and Vibrato
Long Sustained Notes
Practice the notes of a scale with each note being held for 8 counts. Musically that would be two bars with a whole note in each bar and with a tie touching both whole notes. Volume and Pitch much be constant throughout the 8 counts.
Try to increase to 12 counts.
Vibrato
Elvis used vibrato in every song. He couldn’t do those slow songs with excellent vibrato without vocalizing practice.